 Welcome to the module 6, it is hydrostatic transmission systems. This is the first lecture which is lecture number 23 basic concept of hydrostatic transmission systems. Now, here I have described that what we need the hydrostatic transmission. The need for large power transmission in tight space and their control with exceedingly rapid response for military industrial as well as modern application such as robotics requires motors with very high torque to inertia ratio. Now, this with respect to the first paragraph what actually meant that it is always preferable if the output torque is very high whereas, the total inertia of the machines or so to say the weight of the machine is very less. The fluid power units which possesses such characteristics with several orders of magnitude higher than what can be obtained from other conventional units such as electric motor, hydrokinetic units etcetera are still holding the top rank in the field of application in spite of their relatively high cost. If you look into this I have described earlier in the introduction here I would like to say if you consider a ceiling fan you will find that about this motor that is rotating the ceiling fan diameter is 150 millimeter and width is around 50 millimeter. But if you look into the power the power is only about 60 or 80 watt that size of hydraulic motor can give about 10 kilowatt very high torque slow speed. So, this is definitely beneficial while we are using for general purpose where we normally need high torque at low speed. However, if we look into the source then for electric motor power is being generated at a distance may be several 1000 kilometer away and that is being conducted by over a electric where height in some lines and then there is a transformer that is not at the same complex may be outside and you are getting the modified or you say moderate current to use and then you are rotating the fan. Whereas, in case of fluid power although we can have a centralized power generation, but normally it is equipped with the machine. So, if we consider that part probably then the weight is not less, but always it is the end use if you think in terms of end use the motor can be made very small and that can be directly mounted where we are utilizing this torque also the other most beneficial part is that the fluid which is converting the power that can be carried through a flexible pipelines. So, therefore, this is another advantage that means from the generation of the fluid power to the applications point we have flexibility and we can put the motor and the generation source in any directions. Whereas, if we think of say we are trying to we are considering a drive for the industrial truck or may be say any earth moving equipment. If you go for full mechanical system in that case what we have to do we have to put an engine then a gearbox and then the transmission line and they are physically connected with this rigid bodies. So, they have a particular directions there is not much flexibility in placing them within this the whole machine. In case of electrical of course, you can put the generator and the motor in any directions, but with the motor then again you have to connect the transmission line may be through a gear box. So, that part is you cannot put any direction. So, they should have rigid mechanical connections and the size is also very big in comparison to that if you go for hydrostatic transmission then you can put the engineering directions may be pumps and etcetera connected to that fuel a oil tank you can put somewhere else and your motor you can directly fit into the wheel half of the wheel or may be roller in case of the earth moving equipment. Now, the rapid and continuous growth in computer rated control systems such as computer microprocessor interactive control system has kept the researchers active in developing and improving electronic and electro hydraulic synchronous control units. Now, here this is this I would like to mention the fluid power nowadays it is a improve it has been improved a lot because of introducing the control the which is microprocessor or electronics control within that. That means, earlier to this lecture while we are discussing the control of the swath plate in that case earlier it was may be hydraulic control only. Now, along with this hydraulic control you will find that electronic control gives further or better controllability to the systems. So, with that when we think of the such control system using computer and microprocessor then sometimes it requires that we need to improve the fluid power units also to match with this control system or to make it compatible with the control systems. So, this means that while we are improving the control system as well as we have to improve this hydraulic systems also. Anyway, for that you will find that there is continuous development of this fluid power equipment also hydrostatic transmissions also or in other words I would say that hydrostatic transmissions you will find this using the same equipment using the variety in control we can have we can improve the performance. On the other hand it is it has stressed the attention of the fluid power engineers to improve the hydrostatic already I have discussed with a basic knowledge of fluid power and its control as well as that of hardware of conventional hydrostatic units one can enter into the basic design analysis and application of hydrostatic transmission systems. Here I would like to mention we normally use HST actually HST means only hydrostatic transmission. Now we should use the word when we are thinking of the system. So, therefore, we should say HST systems that means the total circuit. Now if we say HST unit then it means the pumps motor actuator etcetera HST unit and HST systems. So, when we mean HST systems then using the pumps and motors bulk etcetera that is a total system. This is already described again I repeat the hydrostatic is used to fluid power transmissions due to the reason that hydro kinetic and hydrodynamic effects that is the fluid inertia effects can be by and large neglected in the design and analysis of such positive displacement units or transmission systems. So, that means HST units or HST systems. A hydrostatic transmission is special case is a special case of energy transmission system ETS where the mechanical energy of the input drives shaft is converted into pressure energy in the nearly incompressible working fluid and then reconverted into mechanical energy at the output shaft. Look at this definition we may be think of that incompressible fluid means we are only thinking of the hydraulic oil or similar fluid which are considered as incompressible. But no here it is meant that whatever may be the fluid even if incompressible fluid to raise the pressure we compress it then we utilize is for converting the energy. This means that this includes also the gas or pneumatics. Essentially an HST consists of hydrostatic transmission consists of a drive wherein the hydraulic energy input element is a pump and the output element is a hydraulic motor. Usually HST pumps and motors are designed and matched to optimize energy transmissions. Actually if you think of the individually pumps and motors then those are designed may be you can in the market you will find different size of pumps and different size of motors. But you will find a particular set of pumps if you use with particular set of motors then you will have the efficient or optimized energy transmissions. Now hydrostatic transmission when will come into a little you will find in most of the cases which are specially designed for hydrostatic transmission. This means that say if you think of the earth moving government if you think of the construction machine in many cases you will find that there is a pump there is a motor there is a valve everything are used together to for a continuous some a typical operations. In that case in many cases both the pumps and motors are designed as a matched part. In some cases you will find their integral the pump motor and a valve manifold where you will find that inside there is a pressure level everything and they are put together. So, this means that usually we have to look into the pump motor design when we are thinking of optimize optimization or optimum energy transmission. Primary job of an energy transmission system is to accept energy input from a source that is a prime mover with its own output characteristics. Look at this we are using a pump we are using a motor for the hydrostatic transmission system. But when we are thinking of driving this pump that is having its own characteristics. So, our output will have the it needs its own characteristics. So, first of all from the prime mover to the output we have to match that characteristics. Now, transmit and modulate the energy within the ETS and deliver an energy output to the load which has its own set of characteristics. To fulfill its job the energy transmission system must have a set of characteristics which permit an optimum match between the prime mover and the load characteristics. An ideal energy transmission system would produce the desired output at any speed irrespective of its input characteristics, but this is never achieved. A gear drive transmission can provide ideal load torques only at a few points where the number of points is equal to the number of speed ratio in gear box over its speed range. For example, you must you must be using the car think of the car transmission. In that case you will often you often say this is a four speed gear box. So, one when there is the high ratio high stress ratio that is for the starting. So, at starting there will be high inertia. So, because the components are accelerating and it is overcoming the static friction. So, in that case you need high torque. So, what is done from the engine to the wheel we provide the maximum transmission ratio of the gears. Now, gradually when it start moving gradually the torque is reducing. So, the situation is improving that the torque has reduced. So, whole you can use the now energy of the engine to transmit a high speed at low torque because the torque requirement has reduced. So, what we do feeling that if it is a manual car we change the gear shift the gear and that transmission ratio is reduced. In that way gradually we reach at a final stage where almost you can say that engine speed is equal to the differential input speed then within differential there may have a slight or one stage reduction. Usually you will find that differential there is a one stage reduction and then differential is there. Now, for the so by gear box what we are doing we are using this engine power at four different speed and each different speed will it is having its own characteristic and we are putting the actual torque requirement within this four different regions. In case of hydrostatic transmission what can be done this can be continuously variable from starting to end. However, there is in that case you may ask that why we do not use the hydrostatic transmission for car the answer is very simple in case of the car the this torque range is very high. That means, when a car is moving at a constant high speed except some inertia even if you consider that except some resistance may be a little resistance and the air resistance there is no real resistance is there. So, we do not need any power we can move, but in case of if you would like to have such a range from the fluid power equipment that is not possible. So, with a fluid power hydrostatic transmission system to have such a range we have to again we have to use a gear box which is not benefited. So, hydrostatic transmission is mostly suitable that if this range of the speed is not very high say let us consider the road or the other earth moving equipment where for the basic purpose for which such machines are designed you will find the speed range is very small. If you if you moving with a roller from one place to other place you will find that it is not benefited it is very slowly moving, but when it is doing its job it is required. By manipulating HST system a closely approximated torque curve may be achieved, but that definitely not in the range of what is required for a car. The primary difference between an HST and a hydraulic system equipped with hydraulic pumps and motor is that an HST is a whole unit in which pump and motor are specially matched to work together. This I have mentioned while I was discussing normally we call the HST system is a the system of the fluid power where the pump and motor are put together and they are matched to achieve a specific purpose. Also HST controls are designed to provide the specific functions to enable the transmission to perform specific task. Hydrostatic transmission offer many important operating features which are remains talled and undamaged under full load at low power loss. This means that we can we can achieve a stall conditions sometimes which is required, but the oil is only circulating within the system. So, power loss will be less this means that suppose say we would actually some some actuator is moving or something is moving and it is transmitting a torque and it is it is doing a job, but what we want that maintaining the pressure it is just holding there it is not moving. In that case definitely we have to raise this pressure and the fluid has to pass through this. So, what can be done in case of variable piston swash plate type pump what we can do we can make this swash plate to it 0 position almost 0 position this means while it is only supplying may be the leakage oil, but with the full pressure. Now, how much the energy is being required for that the because the flow is very small. So, flow into pressure gives the power. So, power is less. So, that is possible with hydrostatic transmission system in case of other we it is it may not be possible say engine and gearbox system this may not be possible. Hold a preset speed accurately against a driving or braking loads. Operate efficiently over a wide range of torque and speed ratio. Operate in reverse at control speed within design limits unaffected by output loads. Transmit a high power per unit volume displacement with low inertia does not creep at a 0 speed, but fine inching may be possible with a little complexity in hydraulic circuit. Now, here I would like to say say let us consider you have you are working with a forklift truck or similar industrial truck. Now, this transmission the moving transmission is may be with a gear box I mean change speed gear box engine etcetera like a car, but here the torque range may be different. However, this lifting etcetera is done by hydraulics. Now, in case of warehouse operation normally what is required you have to lift the load then you have to come back and also for the positioning the load you have to move very slowly with engine gear box the conventional drive it is very difficult. Whereas, in case of hydrostatic transmissions this inching speed is possible only a small displacement it will just move the this truck very slowly. So, that is another important aspect of using hydrostatic transmission system. Get faster response than any other type of transmission the response through fluid high pressure fluid is very quick, quicker than any other system. Provide dynamic braking say if you just close the oil path you will find the whole thing will be stalled in case of high pressure rise of course, it will go through the relief valve, but normal case you will find once you just move the valve close the flow path immediately it will have a braking effect. Provide long life with a little careful maintenance mainly the contamination control here I would like to mention although it is written here, but in most of the cases the failure of fluid power system due to the contaminations. So, although you may not need very much maintenance, but you need to routine maintenance we should do that otherwise this will fail, but with little maintenance definitely fluid power is better and lastly the provide flexible transmission lines that I have explained. Now, if we consider the hydrostatic transmission system again I will repeat hydrostatic transmission system means we will consider the pump motor together including the valve. That means, we shall not consider the pime over which is being used as a input we shall not consider the ultimately what work is it is doing taking the output from the motor, but if we consider the motor output shaft or motor output actuator to the input shaft of the pump the whole unit we will consider as a hydrostatic transmission system. Now, using that system as you say HST's then this can be open circuit discontinuity flow that means it is interrupted flow can be interrupted. First of all another things I would like to mention in normally hydrostatic machines what we call these are all direct current machines that means all the flow output from the pump those are mixed and then this is forwarded to the motor side in motor again this flow is divided into all the pistons. So, flow mixed and then again it is being utilized for the motor. So, we have to call it DC direct current flow. Now, in this direct current if it is called open circuit that means the flow is going to the motor and then it is going back to the tank it is not coming directly to the input side of the pump. So, we should call this is the interrupted fluid. Now, it can be closed circuit also in case of closed circuit what it is the output of the motor flow is the input of the pump directly connected pump output is the input of the motor motor output is the input of the pump. So, simply from this pump and motor hydrostatic unit they again look alike due to the configurations you simply connect input to output to input it should work theoretically. So, this is called piping turn in the piping turn we have open circuit and closed circuit in case of closed circuit it is continuous flow uninterrupted, but actually it is not possible because there will be leakage we will see this how this circuit can be made. Now, again from the control turn each open circuit and closed circuit has open loop that means there is no feedback and there is closed loop which is having feedback similarly for the closed circuit also. So, basically hydrostatic transmission look considering the piping turn and the control turn we can have four basic transmission. Now, the turn open circuit and closed circuit describe how the hydraulic line in the conducting circuit are connected which I have explained in an open circuit the flow path of the fluid is not continuous being interrupted by the reservoir it is going back to reservoir and from the reservoir again it is being pumped in by the pump in closed circuit flow path remains uninterrupted HST may be classified further into two different categories from the employed control point of view which I have explained that is the open loop and closed loop it is sometimes it is termed as closed or closed open loop controls are usually manual or electric hydraulic added manual controls either it may be manual or electric hydraulic added manual controls that means you are basically on of switch and it is being operated, but the closed loop controls include purely a hydrostatic control to all modern control systems. Now, here I would like to mention although it is said that it is hydrostatic control, but the hydrostatic unit itself might be again electronically controlled say for example, we can make such a closed loop control using a proportional valve where the proportional valve is being controlled by electronic systems do not confuse that, but usually the whole unit we would say the hydrostatic control is there. Now, if we try to understand through the symbolic representation representation you see that this is a unidirectional pump and this is an unidirectional motor they are simply connected like this and oil is from the reservoir and oil is going back to the reservoir. So, we should consider this definitely open circuit and also it is open loop without feedback no control is there. So, this is open circuit and open loop. Now, next one is that this is open circuit, but there is a transducer you see this any controls feedback control system or even if it is not feedback what we need that there should have a transducer. Say this is a constant speed or may be constant torque whatever it might be. So, we need a transducer and then there should have the control system and through which the pump is being controlled either for the constant pressure or for the constant speed. Now, third one is that the closed circuit you can see this pump input to motor say motor output to pump input and pump output to motor input like this and you see here what suppose this is the direction of the pump flow out. So, it is going this way and it is coming back this way. So, for which may be this is being rotated anticlockwise and this also the output is anticlockwise like that, but it is possible that here either rotating in the opposite direction or keeping the direction same we can change the direction of the flow and thereby we can have we can rotate in the opposite directions also. Usually in hydrostatic transmission where it is a closed circuit in that case the direction of the pump rotation remain same whereas by changing the swash plate in the opposite directions we can change the direction of the input output of the pump thereby we can rotate the motor in both directions. Consider a forklift truck consider any industrial truck consider any earth moving equipment which is given by the hydrostatic motors in that case transmissions are like that. Engine is giving the output in single directions pump is fitted to that, but varying the swash plate from one side to other side we can change the direction of the flow of the pump thereby we can rotate the motor in the in both directions. And then lastly this closed circuit will have may have also the closed loop feedback this is say this usually this type of transmission are used in many machines where the control is manual a driver is controlling, but in many cases you will find this is having the closed loop control also say if we have some say remote control or may be automatic control for some moving equipments. Say now, nowadays you can think of the guided vehicles automatically guided vehicles in that case also usually we will find closed loop there are many many other cases also where it will be closed loop. So, we here again you can say we can look into this. So, these are the four possible transmission system hydrostatic transmission system. Now, we shall consider a particular circuit now what we find here that this is the pump this is the suction filter first of all let us consider the reservoir is there next this is the suction strainer this is usually 100 to 200 micron in general purpose while hydraulic application. So, this is the suction strainer and then we are having the pressure relief valve here we are having the pressure relief valve this pump we have used the pressure relief valve here instead of using here. Now, the pump fixed or variable displacement this pump it might be fixed or variable displacement in this case what it shown that is variable displacement it might be also fixed displacement. Next we have this motor fixed or variable displacement and there is this is sorry this will be conduit this is not conduct this is this will be conduit rigid or flexible with connectors there we need connectors there then we will have return line filter this is the return line filter. Now, return line filter is usually 10 to 25 micron why it is like that if we put actually we need the well of such filtration 10 to 25 micron even if it is for general purpose, but if we put such an element in the suction line then pump will have it will have additional load and sometimes as these are very poor suction capacity in that case sometimes the suction will fail that is why always we put this 10 to 25 micron filters in this in the return line. Contamination is coming through actually in this case there is a little chance, but if we drive an actuator such and cylinder pistons in that case usually dust comes through the piston rodent. So, we need to have a good quality of filtration there. Now, apart from that there will be control unit then I am sorry there it is perhaps it is a mistake the earlier one if you go back to the earlier one the this is not written that we need a direction control valve this is as I told this is a open circuit. So, we must need a direction control valve to operate this. So, reservoir then suction then pressure relief valve pump motor conduit return line control unit this control unit actually includes this direction control valve also. Now, we shall discuss about a closed circuit hydrostatic transmission. In closed circuit hydrostatic transmission unit the component what we need for open circuit those are there apart from that we need feed or charge pump actually closed circuit means as I have explained pump output to motor input and motor output to farm input. So, ideally we do not need in ideal case we do not need anything simply if you rotate the prime over it will rotate the pump and this pump will rotate the motor, but what happens there is a leakage. So, you will find that initially if it is completely filled with fluid the motor is rotating, but after sometimes you will find that motor is not rotating. So, what is there we need a feed pump this feed pump is look at this is very quite interesting this feed pump it is having a relief valve and then it is going through two non-return valves. Suppose this oil is from the pump is flowing in this directions then it is being rotated. So, definitely this is very high pressure as it is high pressure. So, the ball is closed now this pump is having very low pressure it is only filling the oil. So, what it is it is trying to move in this directions now this is the return line here the pressure is very low. So, oil is going through this now whatever leakage whatever the loss due to the leakage that will be completely filled by this pump. Now if it is try to usually this is fixed displacement, but very low volume now if it is trying to pump additional oil to the circuits it cannot take because these are all say non-flexible conduits we should consider. Although it is flexible, but it is not elastic in that case this oil is going through this pressure relief valve and it is going back to the tank or may be again to this pump here it is connection is so at if it is going through this pump. Now next so adding this feed pump it should work, but look at this this pump is having no relief valve because if I provide a relief valve where then immediately oil will pass through this. So, closed circuit concept will not work, but we need to have some pressure relief valve otherwise if the load is very high if it is exceeds the capacity then in that case the whole system will fail it will burst. So, therefore, this pressure relief valves are there now how it is working we it is called dual shock valve. Let us consider the oil is flowing like this so this is the high pressure line now one pressure relief valve is connected like this. Now if the pressure is high that means it is higher than the for which it is set in that case it will move like this and it will go back to the pump. So, pump will rotate and this oil will circulate a circulate when the oil is flowing in this directions then this pressure relief valve will work. We cannot put a single pressure relief valve so that is why it is called dual shock valve usually either you can put two separate pressure relief valve or this whole system is a integral system a single manifold is used for that. Now here again we have another valve we have used for the purpose of filtration so we can by moving this we can take the oil out. When we are closing if we close this one then oil is first filled and then operation start if we open it oil will come out to the tank for the cleaning purpose. Now this you may ignore to imagine a hydrostatic closed circuit hydrostatic transmission system. To improve the performance and to fulfill other desired task many other appropriate components such as accumulator cooling system special purpose valves etcetera are included in the circuits. Now I would like to mention that suppose you need less fluctuation you can put an accumulator this side as well as this side also and we accumulator works only with the high pressure. So, there should have a bypass circuit also that can be provided here. You need filtered oil for the operation say for example, in case of servo control valve at the utilization side you need to need that oil to be filtered 5 micron or even less. So, in that case a high pressure filter is used in all line. So, all such components can be added to make the circuit more efficient more workable condition. Performance of a HST is not only dependent on the inherent characteristics of the transmission system including the prime mover characteristics, but also on the nature of loads definitely depending on the nature of load there will be variation in HST performance. Usually the performance of an HST means the overall performance of a closed circuit system. When we mean the performance of hydrostatic transmission system we consider it is a closed circuit system. Now what are the type of loads that we should know? There are 4 main classes of loads which in combination can describe most actuation problems. These are called one is friction there will be friction each and every components motor linear actuator everywhere there are frictions. Now then the elastic load that means whenever you are applying the pressure there will be the elastic deformation all the components there. So, that is also we require the first energy goes to expand the elastic bodies friction is there to move the bodies next comes the gravity loads are there. So, that we have to consider and then comes inertia load. Now any load will have some friction and some inertia, but usually one effect is dominant normally it is like that friction denotes any dissipative load which includes rubbing friction along with resistive loads such as electrical generators, propeller drives, fluid trans transfers system etcetera. Elastic loads have force as a function of position only. Gravity loads are constant or quasi constant loads independent of direction of motion such as winches. In case of winch what it is it is moving a load in the upward directions. So, the drum is being winding and so this is a gravity load you are providing this hydrostatic load to move the that is such a load. Now inertia loads are dominated by acceleration effects. So, this load only active when there is accelerations. Now all say if you consider the elastic load, if you consider the inertia load, if you consider the gravity load all are say store energy. Say for example, if you lift a load by winch then you can recover the energy because if you leave the load to fall you can recover the energy except the friction. Friction you cannot recover. So, we should be careful about reduce the friction as much as possible, but other load also suppose the load we are lifting that is not utilized for the recovery of power and to use somewhere, but in some cases you can use that power also. Now if you think of the closed circuits then there are four types of hydrostatic transmission as illustrated here. It can have both pump and motor fixed and then power constant and torque constant. So, this is output speed also constant. All are fixed purpose. Usually you will find that pump is variable drive and then motor is fixed displacement. In that case power is also varying because depending on the motor I mean pressure side we can vary this flow and we can keep this power variable whereas we can keep this torque constant. This system is widely used which is called constant torque system mostly used. You can have also pump fixed motor variable whereas power will remain constant, but torque is varying. So, this is in a sense this is better, but in normal cases we need to have various range to this does not mean we are only a one single torque we are negotiating. It is like that keeping the torque constant you can vary the power. So, this is energy saving then this system constant power system. Now you can make all variable this making all variable means it includes all three. You can keep all fixed. So, it will behave like this. If you keep fixed and make this variable it will behave like this and third is that you can keep this fixed and this variable. So, this system although it is expensive, but if you make this if you design or if you use a system with all variable then you can have all other three features within the system. Now control system may be manual control, pilot operated control, constant horsepower control and constant pressure control. Anything you can achieve you can design the system like that. To determine the actual size of the components all losses must be properly included. That means while you are considering the design it is not that suppose if you need the motor say 20 Newton meter and maximum speed 100. So, if you ideally select a pump of that size you will find exactly matching size you may find that you are not getting the full performance. You have to consider the pump over size why because there are many losses. So, to compensate that losses pump should be of higher size. How much higher size it should be for that you need to calculate all losses together while you are estimating the performance. The losses are leakage losses through the active zones mainly internal leakage such as barrel, valve plate, inner face, active load transmitting contact zones and leakage losses through the slip region. That means there are external leakage say through the piston this oil is going out of the barrel and it is going inside the cases. So, we have to consider such leakage also carefully and this is inter chamber leakage first one and this is the external leakage. Apart from these leakage loss which is proportional to the pressure drop and affect the volumetric efficiency there are few other losses. It can be mentioned here that the design of HST system is a more complex undertaking than when dealing with individual pumps and motors. Actually I would like to say that what is the advantage of the say hydrostatic transmission system which is usually again closed circuit. The advantage is that this has very quick response and return line is not the always with the lowest pressure atmospheric pressure. You may find this pressure difference they maintain in such a way the performance we achieve through such an hydrostatic transmission in far better way than the open circuit. Now, this we should consider the coulomb fractions which is strictly for clogging motion that is that is in case of the pistons you will find at the starting particularly there is a strict motions that we also consider while we are particularly estimating the starting torque of a hydrostatic unit or may be the total system also we must consider the viscous drag. Inertia which is not an actual loss but acts like one during breakaway and acceleration because inertia torque adds to the load and friction torque. You see this initial stage at starting we must have to consider the inertia also. In open circuit these 3 losses are included while calculating the power of motor. In case of closed circuits we have to we can compromise with such loads. Another important features to be accounted is the stall torque characteristics. Usually if you would like to stall say for example, in many cases we need to stop the machine but holding the torque. So, for that characteristics also we should consider because if you would like to hold the load at a particular position your motor may slip. So, that is to be accounted. This is an very important factor in the selection of motor and fluid for transmission and it would force a designer to specify a hydraulic motor up to twice the size. In many cases you will find due to this reason only you have to increase the size of the motor. Now, here I would like to mention for that special purpose motor are also designed. Normal course we use the high speed low torque motors but we can have also low speed high torque motor. Now, another thing is the corner horsepower. What it is called corner horsepower means maximum torque and maximum speed. At that condition we have to achieve. So, every design that is particularly when the motor is at its high speed and high torque what is the power that has to be considered to select the pump including all other losses. So, it is to be remembered while designing a system that these two values do not occur simultaneously. So, if that occurs in simultaneously we need huge power and we get the multiple steps within such hydrostatic transmission system. So, a completely variable control HST is an appropriate device. Now, we can also go for special hydrostatic transmission unit which that first of all what we should do decrease the weight, size and number of movable components. So, low inertia, better startability and increase the displacement per chambers per revolutions. This is the leakage loss, reduced torque, etcetera. Increase the number of power stroke and hence the displacement per revolutions. So, least torque and flow fluctuation, high torque output. So, we can also reduce the irregularities, but in case of motor and hardware it changed to multiply the torque internally. The LSHT low speed high torque motors may be classified according to their internal feature as follows. Now, we can design the low speed height of motor. We can increase the number of power strokes and the increasing the number of pistons or by increasing the number of power strokes for output revolutions. This is say examples are double row, multiple cylinder piston motor, multi low ball piston motor, orbit motor etcetera. Now, case B is that piston area is increased. The best example is the radial piston motor which we have learned earlier a little bit. And third is that combination of A and B. The example is that multi low radial piston motor, multi low radial ball piston motor etcetera. So, this is the end of this lecture. And I would like to say we have not specified any references here because this is the very general knowledge. It is this note is preferred consulting many books. So, no specific reverence for that. Thank you.