8 November 1996

Hydrostatic drive – the

inside story

Materials handler makers Merlo and Sambron have opted for hydrostatic drive instead of the more normal torque converter-based transmission. With help from ZF Great Britains Dave Bramwell, Andrew Pearce roots among a typical systems pipework

HYDROSTATIC drive is one of those things that looks complicated from the outside and, at first sight at least, is nearly as bad from the inside.

But not to worry, as the principles are easy enough. Fundamental to the plot is oil, which is a good place to start – after all, if you let it out the machine goes nowhere, so it must be doing something moderately important.

Oil takes energy from the handlers engine to the wheels. Think of it as an infinitely flexible golden rod, which you can bend to any shape but can never compress. Put a theoretical piston in a theoretical tube and link them to another by oil in a pipe. Push in one piston with a theoretical finger and the other moves out – how come? Energy from your grimy digit raised the oils pressure enough to overcome the second pistons resistance, so it moved. If the second piston was stuck, youd have to push harder – which would raise oil pressure more, and so on.

Thats the general idea behind hydrostatic drive. The handlers engine takes over from your finger, pushing not one but several pistons in a pump. Oil lines connect this to a motor, which also has several pistons and drives the wheel(s).

Resistance at the wheels raises pressure in the hydrostatic system, the motors pistons are forced out by oil from the pump, the motor starts to turn and the machine moves off. Which all seems simple enough but raises a few questions – like how can a piston pushing out cause something to turn, and how can the whole arrangement provide stepless speed variation and forward/reverse drive?

On the way to some answers, three things need to be held in the mind. Within the system, faster oil FLOW produces faster motor speed; more PRESSURE produces more motor torque, and flow DIRECTION determines which way the motor spins.

To see how hydrostatic drive works in practice, its best to look at individual bits then build them into a system. The pump and motor are linked in a largely closed loop, with oil flowing from one to the other and back again. So well start with these major players.

The pump. The pump body is like a revolvers chamber, holding a ring of honed-fit pistons instead of more lethal ammunition. Each piston has a free-pivoting pad at its outboard end, which can slide across a fixed swashplate.

This plate can be angled to set it closer to the pistons at one side of the body than the other – a key point. The pump body sits in an oil-filled housing and is driven round by the machines engine.

Oil to be pumped arrives at the back of a piston, pushing it out and filling the bore. Now, as the pump body is driven round, the piston slides across the angled swashplate and is forced back into its burrow, squeezing oil out through a second port and towards the motor. Each chamber fills and empties as its spun past the ports, so a stream of oil leaves the pump for the motor.

By changing swashplate angle, more or less oil is pumped or flow can be completely reversed. As flow rate controls motor speed, so varying swashplate angle steplessly controls machine speed. Likewise, changing flow direction round the pump-motor loop reverses machine travel.

The motor. This is a mirror image of the pump. Oil from the pump arrives at a rear port and starts to push a piston out. The swashplate is close by the piston at this point but angles away from it, so as the piston pushes outward it slides "downhill" towards the widening gap.

As a result, the pump body turns. Its a bit like sitting down, then pushing with both feet on a slippery plate that angles away from you. If you were sat on a trolley that couldnt move back, you and it would move sideways.

After one piston moves out as far as the swashplate allows, another continues the push so the motor keeps turning. Extended pistons are forced back as the swashplate comes closer, thus emptying their lower-pressure oil through a second port and back to the pump. Flow through the ports is equally good in either direction, so when pump flow reverses the motor is driven the opposite way.

Most motors work with a fixed-angle swashplate, but some – like those in Merlos farm-spec models – angle the plate under electronic or hydraulic control. This increases motor chamber capacity and thus widen the machines speed range.

The charge pump. This does two jobs. While most of the oil churns round constantly between pump and motor at 300-400 bar (4,500-6,000psi) in a closed circuit, some washes through the pump and motor housings to pick up heat, collect necessary leakage from the pistons and keep everything sliding.

Pushing it round a circuit between the housings, a cooler, reservoir and filter is the charge pump, which is usually a gear-type unit driven from an extension of the main pump shaft. This unit lifts pressure to 13-15 bar (195-225psi) before delivering oil back to the pump housing. When the drive system is in neutral (pump swashplate not angled, so no flow) the oil simply circulates through the housings and cooler then back to the reservoir.

But as soon as the driver calls for action and angles the pump swashplate, oil from the charge pump fills piston bores and pushes them out to follow it.

Swashplate control. The pump swashplate moves as the driver works his control lever or (on some machines) opens the throttle. This would be hard work by hand alone, as charge pressure is holding one or more pistons against the pump swashplate.

So a pair of oil-fed servos do the donkey work, using main circuit pressure to boost driver effort, move the swashplate, then hold it at the required angle until another command arrives. As swashplate movement is stepless, so is speed control.

All together now

Bringing the main players together produces a typical hydrostatic drive circuit. It looks complicated, for all the bits are there – the main pump and its swashplate, the servos, the motor and its swashplate, connecting hoses, the charge pump, the cooler, reservoir, filter and control valves.

Not to mention two significant relief valves, provided to limit system pressure should the pump seize or load from the wheels prove too much for it.

Oil wont compress, so pushing the left piston moves the right one out with just about equal force. Oil transfers energy by raising its pressure – the principle underpinning hydrostatic drive (diag courtesy J Deere).

Chief components in ZFs RMF 140 motor. The body (centre, right) holds a ring of pistons (lower centre), whose free-swivelling yellow pads contact the swashplate (silver, lower left). Here, as in most motors, swashplate angle is fixed. In a pump, plate angle is changed by moving the drivers control lever. Oil enters and leaves through ports in the main casing (rear).

Centre: With the drivers control lever in neutral, the pump swash plate is not angled. The body and pistons whizz round past it, but no pumping happens. Top: As the driver moves the control lever to call for a speed and direction, the swashplate takes up an angle. Pistons are pushed out to it by low-pressure oil; then, as the pump body turns, the pistons slide into a decreasing gap and are forced back. Oil is thus pushed out towards the motor, with its pressure determined by the resistance it meets. The greater this resistance, the greater the pressure and the higher the motors torque – up until the main relief valve opens. Bottom: To reverse system flow and thus motor direction, swashplate angle is completely reversed. The pump body is still driven round the same way by the engine, but now flow goes the other way (Diags courtesy John Deere).

Motor operation, or how to turn a straight push into rotation. Top: High pressure oil arrives behind a piston (arrow), forcing it against the angled swashplate. Bottom: As that piston slides down into the widening gap, the motor body is turned and with it the output shaft – see how the yellow square has moved round. As the swashplate gap closes, the piston is forced in and low-pressure oil is pushed back to the pump.

A complete hydrostatic drive system at work. Red is oil at high pressure from pump to motor, blue is return pressure from motor to pump in a closed loop. Orange and light blue are the charge pump/servo circuits, yellow is feed to the charge pump. Engine power turns the pump body (1); the motor (2) drives the output shaft (3). The pump swashplate (4) is angled by the servo (5). Oil flows from the pump pistons (6) to the motor pistons (7), which work against a fixed swashplate (8). Relief valves (9) limit maximum closed circuit pressure. The charge pump (10) draws oil from a reservoir (11) through a filter (12) and fills the pump housing, while a cooler in the return line (13) keeps it temperate.

The pros and cons

Hydrostatic drive scores in several ways. Its compact – the pump can go directly on the engine, the motor(s) are small and can be mounted either direct on the driven wheels or on to a gearbox/axle unit. Its flexible, as pump and motor(s) can be yards apart and connected only by hoses. And its simple to use, with full stepless speed control, dynamic braking and forward/reverse coming from a single lever.

The bad news is cost. Both pump and motor have to be made to very tight tolerances so dont come cheap. Theres also some shortfall in efficiency over a good mechanical drive – though not as much as with a torque converter – and dirt in the system can very quickly bring it to an expensive stop.

The risk here is hidden; although the system always has a filter, only charge circuit oil goes through it. So although all the oil gets there eventually, any dirt in the main closed loop can recirculate for hours. Thats why its vital to keep everything extra clean when working on hydrostatics.