Aerial work platform hydraulic systems: maintenance and repair

In short: the hydraulic system is what lifts, slews and, on many machines, drives an aerial work platform. Most problems start with dirty oil, worn hoses or tired seals. Check the oil level and look for leaks every day, change filters and oil when the machine manual says so and, if you see drift, leaks or strange noises, stop the machine and call a qualified technician.

On an aerial work platform, almost everything that moves depends on oil under pressure: scissor lift, boom lift and extension, turntable slew, steering and, on machines with hydraulic drive motors, travel itself. A well-maintained hydraulic system is therefore a safety matter, not just a productivity one. This guide explains how it works, which signs point to a problem, what maintenance to do and how a repair should be carried out.

How a platform hydraulic system works

The principle is simple: a pump draws oil from the tank and sends it, under pressure, to a valve block. The valves decide where the oil goes and at what flow, and the hydraulic cylinders and motors turn that pressure into movement. After doing its work, the oil returns to the tank through the filter. Pressures are high: in the JLG 450AJ Series II service manual, for example, the main relief valves are set to 220 to 227.5 bar.

Scissors and booms: what changes

On an electric scissor lift, such as the JLG ES series, the lift cylinder is single-acting: it needs pressure to go up and comes down under its own weight. A holding valve fitted on the cylinder keeps the platform in place and stops it coming down if a line ruptures or a leak develops between the cylinder and the control valve. On top of that valve sits the manual descent valve, operated by a cable, which lets the platform be lowered if hydraulic power is lost.

On an articulating or telescopic boom, the circuit is more complex: there are more cylinders (lift, telescope, levelling, jib), a turntable slew motor, drive motors and, on many machines, an auxiliary pump to stow the boom in case of a failure. Every load-holding cylinder has its own holding or counterbalance valve.

Load-holding valves: the part that holds the platform

The European standard EN 280-1, which sets the design requirements for mobile elevating work platforms, requires load-holding cylinders to have a device that prevents unintended movement if an external pipe fails. That device must be integral with the cylinder, flange-mounted directly on it or connected by a short rigid pipe. The standard also notes that a hose burst valve alone does not meet this requirement. In practice: never alter, remove or adjust these valves without the manufacturer's procedure.

Common problems and warning signs

Most hydraulic faults give warning signs before they stop the machine. The table sums up the most common symptoms, the causes that JLG manuals point to and what you should do.

For drift there is an objective criterion. In the service manuals for the JLG 450AJ Series II and the ES scissors, the maximum acceptable platform drift is 5 cm in 10 minutes, measured under the test conditions the manual describes. The manuals also give drift values measured directly at the cylinder rod. Always confirm the value and method in the manual for your model.

Heat causes damage too. The 450AJ manual warns that above 93 °C seals can shrink, harden or crack, which leads to new leaks.

Beware of leaks under pressure

Never look for a leak with your hand. A fine jet of oil under pressure can penetrate the skin and cause a serious injury. JLG recommends using a piece of cardboard or paper and wearing gloves. Anyone hit by a jet of oil should go to an emergency department straight away, even if the wound looks small.

Preventive maintenance of the hydraulic system

JLG sums up the rule in one sentence: contamination is the primary enemy of a hydraulic system. The clearances between parts in pumps and valves are very small, and even a little dirt causes wear. Preventive maintenance is mainly about keeping the oil clean and at the right level, and catching leaks early. For the machine's full plan, see our preventive maintenance guide.

Every day, before work

The operator should carry out the pre-use inspection, as set out in the IPAF checklist and the machine's operation and safety manual. On the hydraulic side:

• Check the hydraulic oil level in the tank, with the machine in the position the manufacturer specifies.
• Look for leaks at hoses, fittings, cylinders and under the machine.
• Check that hoses are secured, with no cuts, no chafed areas and no bulges.
• Do the function test and confirm the emergency lowering works.
• Report any fault to the person in charge and do not use the machine until it is fixed.

For scissor lifts, we have a ready-to-use pre-use checklist.

Filters and oil: follow your machine's manual

Filter and oil change intervals are set by the manufacturer and differ from model to model. To give an idea, this is what two JLG service manuals say:

• JLG 450AJ Series II: change the filter elements after the first 50 hours and then every 300 hours. JLG recommends changing the hydraulic oil every year.
• JLG ES scissors: change the filters every year, or sooner in extreme conditions, check the oil level every 6 months and whenever work is done on the system. JLG recommends changing the oil every year.
• In both cases: examine the used filter for metal particles and use genuine filters.

These figures are only examples from one edition of each manual. Always use the service manual that matches your machine's serial number, and shorten the intervals if the machine works in very dirty or demanding conditions. For oil types, mixing oils and contaminated oil, see our hydraulic oil and lubricants guide.

Hoses and fittings

• A hose with a leak, a bulge or exposed steel braid gets replaced. It is not repaired with tape or a clamp.
• Replace the complete hose assembly, with its end fittings, to the original specification: pressure, bore, length and fitting type.
• Correct the routing and clamps so hoses do not rub on the structure or twist when the boom moves.
• Clean the area around fittings before opening the circuit, so no dirt gets in.
• ISO/TS 17165-2 brings together good practice for installing, inspecting and replacing hydraulic hose assemblies.

Take care when cleaning the machine too: the 450AJ manual says never to steam-clean or pressure-wash hydraulic components, because water and dirt can get past the seals.

Repair: when to repair and when to replace

Many hydraulic faults are fixed by replacing the part that failed: a hose, an O-ring, a cylinder seal kit, a solenoid coil or a filter. Others call for a whole component to be replaced. A few criteria help to decide:

Genuine parts and qualified technicians

JLG manuals require replacement parts to be identical or equivalent to the original parts. In the hydraulic system this matters even more: seals, valves and hoses must withstand working pressures, and holding valves are part of the machine's safety system. In Portugal, Decree-Law no. 50/2005 requires employers to ensure adequate maintenance of work equipment and, where one exists, to keep the maintenance log up to date.

Safety before opening the circuit

1. Lower the platform and stow the boom or scissor whenever possible.
2. If you have to work with the machine raised, block the structure: safety prop on a scissor, support or slings on a boom, as the manual specifies. Never work under a boom or scissor that is held only by the oil.
3. Switch off the engine or power, remove the key and isolate the machine as per the manual.
4. Relieve circuit pressure before loosening any fitting or component.
5. Wear gloves and eye protection, and keep absorbent material at hand for spilt oil.
6. Cap open ports and lines with plugs so no dirt gets in.
7. At the end, check the oil level, run several full cycles of each function, confirm there are no leaks and test the emergency lowering before returning the machine to service.

After a repair that affects lifting, load holding or stability, the machine should be checked before it goes back to work. Find out what the law says about periodic checks in our guide to mandatory aerial work platform inspections.

How Trapézio de Prismas can help

We are the official JLG dealer in Portugal and provide technical assistance and repairs for aerial work platforms. If you notice any of the signs in this guide, such as leaks, drift, slow functions or pump noise, request a technical inspection from Trapézio de Prismas.

You can also find filters and parts and JLG manuals. If you have any questions, contact us.