There are two basic types of floor jacks that you will find inside a typical garage; mechanical and hydraulic. Lets look at each of these separately.
Mechanical Floor Jacks
Mechanical floor jacks use a screw mechanism to apply force to the jack body and lift the vehicle. The short thread distance (or thread pitch) of the screw gives a mechanical advantage allowing heavy loads to be raised with moderate effort. The trade off is that the screw has to be turned many times to achieve a small amount of lift. These are the type of jacks that you find in your vehicle’s roadside assistance kit.
Hydraulic Floor Jacks
Hydraulic floor jacks use valves, hydraulic pistons, and multiple different levers to gain mechanical advantage and lift the vehicle. The handle of a floor jack is positioned so that the hinge gives the user a tremendous mechanical advantage right from the start. For every pound of force that is applied to the floor jack handle, an amount of force exponentially greater is applied to the hydraulic piston system. The hydraulic system consists of a reservoir, two cylinders of different diameters and various flow control valves. Mechanical force from the jack handle is applied to a small hydraulic piston which is then amplified into a much larger force due to the difference in the diameters of the hydraulic chambers. How do this happen?
A fancy law of fluid dynamics called Pascal’s Law says that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same. HUH! OK, basically it means that when you force fluid from a small chamber into a larger chamber that the pressure in both hydraulic chambers remain the same. However, and this is the important part, in the large cylinder, the pressure is being distributed over the larger surface area of the larger piston which creates more force. Pressure is measured in units of force per area such as pounds per square inch, or psi. So the more square inches of surface area you apply the pounds to, the more force you get.
When the floor jack is in the lifting mode the valving system is considered to be closed (typically the valve is closed by twisting the jack handle to the right). In this mode the jack functions as follows:
- The jack handle is raised and fluid is drawn from the reservoir through a check (one-way) valve into the small hydraulic cylinder.
- The jack handle is pushed down and the pressure builds up in the small cylinder forcing hydraulic fluid through another check valve into the large cylinder.
- The pressure created in the large cylinder moves the large piston with an amplified force. Note that although the output force is higher, the distance traveled by the large piston is considerably less than that of the small piston. This is the reason the jack handle has to be raised and lowered many times to raise the jack only a short distance.
- The piston of the large cylinder is attached to the lifting arm in a manner that creates another lever to further increase the mechanical advantage and lift the saddle in a horizontal fashion.
The floor jack will stay in the raised position and maintain hydraulic pressure as long as the valve is in the closed position and the piston seals are in good working order. Note that it is not advised to leave a vehicle in the raised position on a hydraulic jack for any length of time and you should never work under a vehicle supported only by a hydraulic jack (see safe lifting guide). The best practice is to lower the vehicle onto a jack stand placed under the axle or lower control arm.
When the vehicle needs to be lowered back to the ground, the valving system is opened (typically by twisting the jack handle to the left) and the fluid in the hydraulic pistons flows back into the reservoir which allows the large piston to retract into its bore and lower the lifting arm .
Well, there you have it. I hope that this was informative and that you now have a better understanding of how floor jacks work.