of the trade

17 October 1997

Hydraulic controls – spools

of the trade

The goings-on below your

telehandlers control levers

are explained by hydraulics

specialist Peter Pridham,

chief of component and

system company Norsales.

Andrew Pearce passes on

some fluid wisdom

AS EVERY journey starts with a single step, so getting a telehandlers rams underway begins at the drivers hand. Or rather brain, for here lives the infinitely subtle loop linking eye, ram, muscles and lever.

Human engineering aside, what actually happens when you push that lever or move a joystick? Ultimately a spool valve shifts inside its block, channelling high pressure oil to (or from) the appropriate ram. Its how that spool is moved that concerns us here.

March of time

Not so long ago everything was simple. Spool blocks lived up in the cab with the driver, each sprouting its individual control lever (Fig 1). Control with these is direct and proportional; the more the operator moves the lever, the more the spool shifts and the more the ram moves. High pressure oil comes into the spool block and leaves it for the ram in a system that is cheap, simple but not very pleasant for the driver if a pipe bursts.

Legislation eventually put an end to high-pressure oil lines in the cab. So spool blocks shifted outside with cables taking cab lever movement to them – just as with the remote valves on hedge-cutters, root harvesters and such (Fig 2).

Things got a little more complex when drivers wanted to work two services from a single lever. For this, dual axis cable controls came along (Fig 3). Each cable connects to a separate external spool block.

Moving the lever forwards and back operates one cable to and fro; moving it side to side operates the other. For safe road travel or specific jobs such as man platform work, some versions let one or both movement planes be locked mechanically.

Time moved on. Users not unreasonably wanted to work a third or fourth service from a single stick, so these started to sprout buttons (Fig 4). Each makes or breaks an electrical circuit, opening or closing a remote solenoid valve.

Oil can thus be easily diverted to another machine service, channelled to a motor or allowed to operate a ram. But a mechanical switch is either off or on, letting oil flow until the switch changes state. Thus control of such secondary stick functions is not proportional.

Now, cable operation has a lot going for it. Its relatively cheap and brings proportional control. Cables themselves are easy to bring into a cab, can put up with reasonable bends and will withstand a certain amount of agriculture. But it could be more durable.

At a guess, a farmer using a handler on stock work for an hour a day makes around half-a-million control movements every year.

Cable controls put up with that level of use for a while, but sooner or later will wear. Slop develops, reducing precision. And if a worn cable is left to deteriorate and one day sticks, safety can be compromised.

So where can you go from cables? To a pilot control system, where oil takes over the job of pushing and pulling the remote spool about.

Oil is incompressible. If you push one end of a column of it, movement appears directly at the other. Thats what pilot control is about; under the systems hand lever or two-axis joystick, mini-spools control the flow of low-pressure oil (Fig 5). This oil is piped into the cab from the main hydraulic system or a secondary dedicated one, and, when the control lever is operated, exits to tiny hydraulic cylinders on the ends of the external high-pressure spools. These in turn control the telehandlers rams or whatever.

Move the control stick and a mini-spool opens in direct proportion. A column of oil pushes the remote main spool across and the service operates. Move the lever the other way and low pressure oil goes to the other end of the main spool, shifting it back and reversing the service.

If the handlers hydraulic systems allows, shifting the joystick diagonally opens two mini-spools at once and makes two services – for instance lift and crowd – work simultaneously.

Pilot control is proportional. Precision is high as oil has no "give" in it, safety is good as only low pressure lines are in the cab, theres very little to wear out and the sealed service is practically bulletproof. Cost, though, is significantly higher than using cables.

And while pilot control is fine for kit like hard-worked telehandlers and in systems where response times need only to be moderate, in forestry – and increasingly farming – shorter control response time can improve machine/operator productivity significantly. Guess what that takes?

Flying by wire

Electronics can speed response by a factor of four. There are currently three main ways to use the speedy microchip – on/off electronic switching, proportional switching and complete process control. In each case, the remote spool valve is opened and closed by electrical solenoid. Theres no cable or oil connection from the cab, only wire.

Electronic on/off switching lets remote solenoids be turned on and off up to 300 times/min, potentially saving time and improving operator output beyond the abilities of mechanical controls. Its used increasingly instead of relays and older hard-wired systems.

Proportional switching lets particular types of solenoid – different from the ordinary on/off variety – be moved gradually under joystick control, rather than being turned on or off like a light switch.

Below the joystick, electrical resistors steplessly change value as the stick moves. As a result, voltage "seen" by a control unit shifts in tune with stick travel. The microchip controller simultaneously generates an output signal which flits by wire to the spool solenoid.

Start with the joystick in neutral. Theres no output signal from the controller and the spool valve is shut. As the driver shifts the lever to, say, raise the boom, joystick resistors change value and the controller starts generating a pulse stream.

First to arrive at the remote solenoid is a controlled thump of power, which starts the spool opening. Then, depending on the size and frequency of the subsequent pulses, the spool valve takes up a station corresponding to joystick position and oil flows to the ram accordingly.

Always ready

To make sure the spool is always ready to move on, a stream of tiny pulses keeps it gently jittering. When the operator shifts the stick to lift the boom faster, the controlling signal gets larger, the solenoid exerts more pull and the spool opens further. When the stick comes back to neutral, the spool closes.

As with pilot control, diagonal joystick movement can work two remote valves at once.

Proportional switching is already with us in telehandlers, hedge-cutters, loaders, combines and the clutch pack organisers of some tractors. The coming thing for farming – though its been around a long time in industry – is process control.

Well look at this only briefly as it has more to do with external equipment than telehandlers. Say you want a machine – a reversible plough, a bale wrapper, bale stacker or similar – to go through a pre-set routine, all by itself, once youve started it off. This sort of automatic sequencing makes sense, as it takes load from the operator and potentially improves work rate.

To do it you need an in-cab control box, a computer processor and a dedicated block of solenoid spool valves on the machine (Fig 6). Oil can come from the tractor or a self-contained machine supply.


The computer is pre-programmed with the sequence of operations and given a time limit for each. Once the operator starts things off it runs through this ladder of steps, opening and closing solenoid valves on cue and thus sending oil to various areas as needed.

Confirmation of where in the sequence different bits of the machine are comes from switches or sensors, so the computer cant initiate step B until A has finished, and so on. Add an emergency override switch and the ability to move stepwise through the sequence by manual control, and you can see how a lot of tackle will be managed before long.

Norsales (01302-330303) specialises in hydraulic control systems.

A single joystick control has led the popularity stakes over the multi-lever control system originally found in telehandlers.

Fig 1: Simple stuff – a direct operation spool carries its operating lever on the block.

Fig 2: As legislation moved high pressure oil lines outside the cab, cable operation took lever movement to them.

Fig 3: Dual axis controls use two cables to work two remote spools. The locking tab (red, centre) physically blocks lever movement for transport or specific jobs.

Fig 4: Buttons and yet more buttons. Each gives an on/off signal to a remote solenoid valve, adding multiple functions to a simple cable joystick.

Fig 5:Below a pilot control joystick, mini-spools direct the flow of low-pressure oil. Benefits over cables include better durability and more precise control.

Fig 6: State of the art. A control sequencing system uses computer power to open and shut remote solenoid valves, in a pre-set order and for a pre-set time. The driver uses an in-cab panel (front), while the big block (rear) holds the filter, the valves and mounts on an implement.

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