QUICK FIX
Your four-hour workshop
Belligerent? Incontinent? Unable to get it up? Then it’s time to overhaul your jack, says Theo
EXPLAINER: TROLLEY JACK PRINCIPLES
Vehicles are heavy, sometimes weighing in at more than two tonnes, so lifting one as a puny human requires mechanical and/or hydraulic leverage. A hydraulic trolley jack uses both. A long jack handle operates a small hydraulic piston about a fulcrum/lever that provides a large amount of mechanical leverage. The small piston acts on incompressible hydraulic oil, which acts upon a larger piston, providing liquid leverage: the small piston has to move a much further distance (in multiple, small increments) with lower force in order to move the large piston a short distance with great force.
JACKS IN PRACTICE
The hydraulic system in most bottle and trolley jacks is very simple. The ram, containing the large piston, is single-acting, so it is only powered in one direction, the weight of the vehicle returning the piston. One-way ball-valves prevent the large piston from pressurising the small one, allowing the small piston to act like a hydraulic ratchet, a manually operated valve releasing pressure to the reservoir when lowering, and a pressure-relief valve preventing damage when fully extended. Some mechanical advantage is lost in raising the arm, but the overall advantage is still significant.
START: ASSESS WORK 15 MINS
1 Wet bits
The actuating piston can leak, as evidenced by wetness on top of the valve block. The release valve and other drillings are sealed with O-rings that can also leak.
2 Dribbly bits
A sinking jack is likely to be caused by oil bypassing the piston seal, which will eventually find its way out past the rod seal, leaving tell-tale marks underneath, as seen here.
TOP TIP
Don’t have a funnel? Run some oil down a screwdriver.
3 Not very rampant?
If the jack fails to reach its maximum height and the action goes flaccid, it’s probably running low on oil. A temporary fix may be to top it up and bleed it, but it’ll need work soon.
4 Ergonomic aggro
You need to be able to control the descent of the jack, which is
