Hydraulics are all around us, and the most common piece of hydraulic equipment most of us interact with is the humble jack. Whether you’re talking about small and compact bottle jacks or the wider-based trolley jacks, though, these amazing machines can still hide a few surprises…
1) Though most use oil today, hydraulic jacks were once filled with water.
This one isn’t all that surprising – they clue is in the word ‘hydraulic’ after all. Most use oil as a hydraulic fluid today because it is easy to use and actually protects the moving parts of the equipment – something that can’t be said for water. Water was originally used because of its sheer availability and the expense of refined oil products before the modern era.
The fact is that any incompressible fluid could be used to operate a hydraulic jack… with varying levels of convenience. Something like mercury would function, but it would be prone to leaking poisonous gas. You could even use mayonnaise in a pinch, but I struggle to imagine a scenario where that would be necessary. Oil or water really are the most obvious choices.
2) The power of a hydraulic jack is a function of the width of its cylinder, not its length.
The pressure of a hydraulic system is limited by its weakest part, typically the pipes or hoses which transfer the hydraulic fluid from the pump to the piston, or the places where different parts join up. However, the force of a hydraulic jack is equal to the pressure of a system times the surface are of the piston head. Even if you can’t realistically increase the maximum pressure inside the jack, you can double its actual lifting capacity by doubling the area of the piston head.
The length of the cylinder is still important, though. That is what controls the actual distance the hydraulic jack can lift the load. If the piston can only extend 10 cm, then you’re never going to be able to lift he load more than 10 cm with that jack.
3) Commonly available hydraulic jacks can lift loads of up to 150 tons, sometimes more!
The only real limit on lifting capacity is the size of the piston – and, in all honesty, the strength of the thing you are lifting – always make sure you are using a designated, reinforced jack point. Check your car’s owner’s manual!
Archimedes is supposed to have said ‘Give me a place to stand and a lever strong enough and I can move the world.’ Perhaps today he’d be asking for a wide enough jack –though it would be the size of a continent at that rate. He’d still need a place to stand, as well. That would be even more complicated.