Hydrostatic Drive Would It Pay?
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AlthoughFluidTransmission might not Improve Fuel Consumption, it has certain Important Advantages By P. A. C. Brockington,
A.M.I.Mech.E. IN a number of manufacturing processes the availability of an infinitely variable drive has improved production efficiency by at least 30 per cent. Power is normally derived from a constant-speed electric motor, and the rate of production is partly dependent upon modifying the torque and speed characteristics of the drive automatically to match cyclic or spasmodic variations of
load. .
This often represents a far more complicated problem than providing a range of ratios in a vehicle transmission to enable the engine to produce its maximum torque and power at various road speeds. Industrial drives have been highly developed in the past 20 years, and include a number of hydrostatic types which give a favourable transmission efficiency under arduous conditions.
The case for hydrostatic vehicle transmissions can be argued far more convincingly than was possible when its adoption was reviewed by vehicle makers before the war. Since then, however, the increased capital cost of production plant has further prejudiced the introduction of unorthodox systems. It has also increased the economic penalty resulting from minor design faults.
Despite the many proven advantages, of hydrostatic transmission and the promise that the efficiency of an automotive unit would he acceptable, its development f a r quantity produced vehicles is therefore unlikely unless extraneous factors favour its adoption.
In conformity with the need to facilitate the work of the driver, the use of power-operated systems for brakes and steering gear is gaining in importance. This may be regarded as one of the "extraneous influences" which could promote the development of hydrostatic al 2 transmissions, and the necessity to provide a secondary braking system in addition to the wheel brakes might be another factor of importance to its future.
In many cases, the relative simplicity of storing power in hydraulic accumulators would offer an immediate advantage, or promote the development of additional power-operated equipment to increase the scope of mechanical-handling aids.
Increasing use of hydraulic motors to drive auxiliaries is indicative of their potential, a particular merit of the system being a negligible wear factor. This would also apply to hydrostatic transmissions, given that the fluid were adequately filtered.
The type of system envisaged as the most suitable for vehicle transmission is one incorporating a variable-delivery hydraulic pump and a hydraulic motor of a similar type. A particular feature of hydrostatic drives is that the pump acts as a variable-capacity motor on the over
run, which offers a valuable source of braking power. In most types of pump. fluid flow to the motor can be reversed without using an auxiliary system.
With due regard to the reduction of pipe length to a minimum in the interests of efficiency, the power unit and pump may be placed in any position relative to the hydraulic motor or motors to suit the requirements of the designer.
If a vehicle were equipped with hydrostatic transmission, it would be possible to eliminate the gearbox, propeller shaft. and even the differential and final-drive gearing, and some authorities are confident that if the system were specifically designed for automotive applications it would give a substantial reduction in vehicle weight.
Braking Effort
Judging by established types of industrial hydraulic drive, it might also be possible to offer a standard unit with a larger torque range than that provided by conventional gearboxes without materially adding to the size and weight of the hydraulic components. The braking torque ,available would equal that normally associated with the lowest ratio in the gearbox, and, of equal importance, the braking effort would be smooth.
Another useful feature of hydrostatic drive is that the relief valves can be adjusted automatically to limit the maximum torque output, which could be of benefit to tipper operators and others engaged on rough-site work.
There would he no counterpart of the clytch, the output being reduced to zero when power was not required, as distinct from disconnecting input and output members. In some designs, relative movement of the pump parts in the " neutral" position is restricted to a simple ring member, and drag is negligible.
le most straightforward layout would 3ne in which one motor was used, 2n by a pump in the same casing, the being mounted in the normal gear
position. This would eliminate ping losses in pipe connections, and Id have the immediate advantage it would enable existing chassis ily to be converted to hydrostatic ;mission.
hydrostatic transmission were fully oited, however, it is probable that a rate motor would be employed for
driven wheel to obviate the use of -drive gearing and a mechanical rential. Also of importance would se facility with which the fluid system d be designed to improve wheel ion by limiting differential action.
se obstacle to employing separate :s would be the necessity to equip vehicle with independent rear susion so that the motors could be ly attached to the chassis, or natively to use high-pressure flexible . Attaching the motors to an unng member would be acceptable with -d to the operation of the mechanism, direct wheel mounting would reduce werall weight of the axle, but there ,me doubt whether dependence on 31e pipes would be satisfactory.
particular merit of mounting the p in unit with a single motor is depending on the type, it is then ble to provide direct drive for top running without relative movement the components. In this case. ency in top gear might be higher that of a conventional gearbox use of the absence of layshaft drag. ould the overall efficiency of a ostatic drive be adequate for vehicle mission? This is the first considera and no development engineer would ipt to evolve a suitable system unless 'ere satisfied that it would afford a consumption rate comparable with returned by vehicles with conven1 transmissions.
dging by the majority of established ;trial drives, the potential efficiency .sents a borderline case; a small Jrable increment in efficiency could the system unassailable advantages.
Free-wheeling Action four-wheel drives and some multiler applications, a simple control : could be incorporated in the system isengage the drive to one pair of Is and to provide free-wheeling n. This advantage is indicated by atent taken out by the Rover pan y (The Commercial Motor, L 25, 1958) in which is described a tulic-mechanical transmission for wheel-drive vehicles.
Araulic operation is applied only to 'ront wheels, the rear wheels being n in the normal way. An engine n pump supplies fluid to two indeent wheel motors arranged in series, delivery is controlled by a two-way which can be used to by-pass the to the intake side of the pump.
e system also incorporates a onevalve located in a pipe connection between the motors in the high-pressure side of the circuit; when the valve is open the fluid can circulate freely through the pipe and this allows the motors to free-wheel.
A significant implication of this patent is that hydraulic front-wheel drive has possible advantages, in the opinion of the patentees, with regard to first cost and weight. Controlling the speed of the motors by employing a by-pass valve in the pump circuit has the merit of simplicity, but the volume of fluid delivered by the pump would remain constant at a given speed. The amount of fluid by-passed would represent an avoidable efficiency loss.
Not mentioned in the patent is the reduction in frictional losses which would result from draining the motors when extra traction was not required. This could provide a worthwhile gain in economy, particularly in cold weather, One of the disadvantages of employing a front-wheel-drive vehicle for short runs on good roads in the winter is that the rotating gears of the front system continue to absorb a substantial part of the engine power output when the drive is disconnected.
Of related interest to the Rover patent, the Serck-Bchr hydrostatic drive evolved for the control of the cooling fan also incorporates a by-pass system with which the speed of the motor (and fan) can be regulated by means of a valve. Although this is an auxiliary drive, it is of additional interest because both the pump and motor units are of the multiple-piston type, a feature common to many large industrial drives.
More Favourable
When the pump is operating on maximum load, and there are no friction losses resulting from by-pass circulation, the efficiency of both pump and motor is 96 per cent., which is a much more favourable figure than the maximum efficiency of most industrial-drive units. The part-load efficiency is at no time less than 83 per cent.
If a system of this type were applied to the transmission of a motor vehicle and separate motors employed remotely from the pump, losses caused by fluid friction in the pipe connections would have to be taken into consideration. These losses could, however, be reduced to a minimum by employing large-bore pipes.
A feature of the positive-displacementpiston layout is that a combined pumpmotor can readily be designed to give direct drive without pumping losses, the two members being locked together. A good example of this type, the Carter variable drive, also enables a proportion of the power to be transmitted by direct torque between the two units throughout the speed range.
In this case the pistons arc arranged, radially, and control is provided by eccentric members in both units. The range of variation covers ratios up to 27 to 1.
In the past 10 years the design of hydraulic accumulators has made great progress, and building up a reserve of power for operation of brakes and auxiliary gear in the event of engine failure does not present a problem.
The possibility of conserving power by regeneration on the overrun has represented an ideal from the early days of the motor vehicle, It is unlikely that a practical means of power regeneration by hydraulic means could be developed to enable stored energy to be used to supplement the power of the engine during normal runs on the road.
It might, however, be feasible to use large lightweight hydro-pneumatic accumulateas (possibly box-type or tubular chassis members could be employed for this purpose) of sufficient capacity to shunt the vehicle short distances in the depot and to provide power for stationary auxiliaries. They could also be employed to start the engine. A smaller battery could then be used, and this advantage, combined with elimination of the electric starter, would partly or wholly offset the extra weight of the accumulators.
Sliding-vane Type
Another industrial design that might be applied to vehicle transmissions is the sliding-vane type, the vanes being normally mounted in a casing which revolves with the rotor but which can be moved radially to vary the eccentricity of the assembly and the pump output. In the concentric position, there is no relative movement between the vanes and the rotor, and the pump is inoperative.
This would correspond to the neutral position of the conventional transmission. Reversed drive is obtained by reversing the eccentricity of the pump casing relative to the rotor. Industrial vane-type units include the Churchill-Sturm variable drive.
Least complicated of all the units, the rotary pump, can be designed to operate efficiently at high speeds, an average efficiency of over 85 per cent, being claimed by one manufacturer for both pumps and motors. The favourable highspeed characteristics of the rotary pump could be art all-important factor in its application to vehicle transmissions.
In the design of Kcelavite pumps and motors, the units operate on the " rotaryabutment " principle. The main rotor incorporates two projecting axial blades, and a sealing rotor has two corresponding recesses, the two shafts being geared together.
Abutments between the recesses of the second rotor act as seals. There is no contact between the two members and the rate of wear is negligible. In a typical layout, a variable capacity unit is employed as a pump and the motor is of the constant-capacity type. Capacity is varied by axial displacement of the main rotor.
The possibilities of the positivedisplacement system based on variablestroke pistons are given further weight by the development of the Dowty hydrostatic drive for tractors, which was demonstrated at the Smithfield Show last December and described in The Commercial Motor on December 6, 1957.