How a Trolley Jack Works - and How to Use One

Let's take a look at how a trolley jack actually works and how you go about using one safely to get your vehicle raised for whatever task you wish to undertake.

It's true to say these jacks work by lifting a vehicle when the hydraulic system goes to work as a result of the operator pumping the attached metal handle up and down. That's the simple way to describe the operation.

However, there's a lot more going on if we home in a bit on the specifics of the actual procedure. The following will sound pretty technical, bit we feel it's important to explain exactly what's taking place when one of these excllent lifting devices goes to work.

All car jacks are devices which multiply relatively small applied forces which are initiated either mechanically or hydraulically, to move, lift, or support heavy loads. 

Force flows from the person applying it to the vehicle to be raised, by means of either a hydraulic cylinder - as is the case with a trolley jack - or a screw-thread mechanical device such as a standard scissor jack. 

The broad bases on trolley jacks helps to ensure they do not tilt sink into the ground under the weight of a vehicle. These jacks also have stop-point safety valves to prevent them attempting to exceed the weight limit they are rated for.

A hydraulic jack transmits a small amount of force applied at its handle socket to a lifting arm - the force with which the lifting arm moves is much larger than the initial force. This in turn allows a small force from a small mass to do work against a larger force or larger mass.


The component responsible for accomplishing this transmission is called a hydraulic cylinder. To be accurate, there are two cylinders involved - one larger than the other. The force from the socket handle pushes down on a piston of the first, smaller, cylinder - increasing the pressure inside the cylinder. 

This then displaces the hydraulic fluid inside which starts to move through a discharge check valve into the second, larger cylinder. At the same time, the pressure of both cylinders equalizes. 

When the piston is pulled back again the discharge check valve closes as a suction valve ball opens to draw oil back into the pump from a reservoir. When the operator pushes the jack's handle down again, the suction valve closes and the discharge valve opens again, displacing more hydraulic fluid into the larger cylinder. 

As fluid flows into the second cylinder a larger piston is allowed to be displaced by the incoming fluid, creating the necessary force for the lifting arm to operate. Although pressure is the same in each cylinder, the force passing through the lifting arm is much greater than the initial force applied to the pump due to the larger and smaller piston diameters being different, based on the circumference of their respective cylinder. 

This is known as hydraulic multiplication and it's what allows a relatively small amount force applied to the actuator to lift something as heavy as a vehicle.

A jack will remain its raised position under hydraulic pressure providing the valve system remains closed. When the operator want the jack to be lowered again, it simply a case of opening the valve system. This is usually carried out by twisting the jack's handle to the left. The fluid from the pistons will then return back into the reservoir.

Phew, if you found that all a bit mind-boggling, don't worry - it might all make perfect sense to an expert mechanic, but many of us it's all a bit too technical. So it's probably best not to start talking about Pascal's Law - this relates to fluid dynamics and pressure, and trolley jacks are a good example of Pascal's Law in action.  

What really matters is that the procedure described earlier results in a trolley jack doing an amazing job at getting a vehicle off the ground, with the minimum of exertion required on the part of the operator. 

How to Operate a Hydraulic Trolley Jack Safely and Efficiently

We'll try to keep things simple now as we explain how to use a trolley jack for the first time after you buy one.These are only general guidelines - you should carefully follow the advice that comes with a specific jack's instructions leaflet or owner’s manual.

We'll assume you have already ensured the jack has been filled with the correct amount of fluid for the hydraulic system to work correctly and you have bought a jack that has an adequate weight capacity rating to raise your vehicle.

First, you should ensure your is parked on a completely flat area - there should be no slope or unevenness whatsoever. Put your vehicle in the transmission 'park' or first gear on a manual transmission vehicle, and put the handbrake on. You should also consider placing wooden or metal chocks on wheels to totally rule out any risk of rolling.

If you have never used a trolley jack before, it's a good idea to try raising and lowering it a few times before actually using it on your vehicle, just to gain a bit of confidence and get the feel for how it operates.

Next you need to locate the most suitable jacking point on your vehicle and, if you plan to carry out work under your vehicle, decide which axle stand support points you’ll use on the vehicle after it is raised. If you don’t know for sure, consult your vehicle's manual or find out the information you need online. Jacking points are normally located at a major point on the vehicle chassis, an axle unit or a cross member. If you get this part of the operation wrong you run the risk of damaging your vehicle - or worse still, injuring yourself.

Axle stands are essential accessories as a trolley jack is only meant to raise a vehicle and not meant to bear its weight for any length of time.

Next you need to attach the long trolley handle to the jack's release valve and turn it tightly in a clockwise direction to close the valve. Then you need to wheel the jack into place below your vehicle with its saddle positioned immmediately below the jacking point, ready to be raised to make contact.

Next you need to pump the handle up and down until the jack's saddle makes contact with the jacking point. Double check below your vehicle to ensure the saddle is positioned accurate and in a stable way. As you continue to pump the handle the jack will start to bear the load of the vehicle as it will gently start to lift off the ground until it reaches the required height. Next you should place axle stands below your vehicle at adjust them in height as desired.

When you are finished working on your vehicle you should initially raise the jack slightly to be able to remove the axle stand. Then you can lower the device slowly and gently - usually by turning the long handle to the left. This activates a valve which will relieve pressure from the hydraulic cylinder. Do not allow the vehicle to lower too quickly or you could cause damage.

That's it - it's not a complicated procedure. After you've used a trolley jack a few times your confidence will grow and it will become an essential piece of equipment for keeping your vehicle in tip-top condition.