# Problem: Car Hoist

How many SCF will it take to fully extend the cylinder if the air/oil tank is full of oil when starting? How many SCF will it take to fully extend the cylinder if the air/oil tank is only 1/2 full of oil when starting? The supply tank has 1-1/2 times the volume of the cylinder. Cylinder has a 10″ bore and an 8′ stroke. Working pressure is 100 PSI.

##### See the Solution

This problem is to show the additional air consumption needed when the air over oil tank is low with oil that is used in car garages for the car hoist.

Solving with a full tank of oil:

Area of cylinder: 10² x .7854 = 78.54 sq. in

Stroke is 8’      8’ x 12” = 96”

Volume of cylinder: 78.54 in² x 96 in = 7,539.84 in³

Cubic feet = 7,539.84 / 1728 (cu. in. / cu. ft.) = 4.36 cu. ft.

Compression ratio (C.R.): 100 + 1 = 7.8 C.R.
14.7

SCF: 4.36 CF x 7.8 = 34 SCF when the tank of oil is full when starting to lift

Air/Oil Tank is only half full of oil when starting. We now have to pressurize the air/oil tank before the hoist will start to lift

Volume of cylinder is 4.36 CF
Volume of air/oil tank is 4.36 x 1.5 (size of air/oil tank) = 6.54 CF
Half full of oil means we need an additional volume of air that equals
6.54 /2 = 3.27 CF

Volume of air needed is: 4.36 (cyl.) + 3.27 (tank) = 7.63 CF
SCF = 7.63 x 7.8 C.R. = 59.51 SCF

Note: If the tank of oil is only half full, 59.51 / 34 = 1.75 times as much air is needed every time the hoist is fully raised.

### Deadline past. Not available for submissions.

By Ernie Parker, CFPAI, CFPSD, CFPS, CFPMM, CFPMT, CFPMIP, CFPMMH, CFPMIH

This teaser is printed in the Fluid Power Journal. Those who submit the correct answer before the deadline will have their names printed in the Society Page newsletter and in the Fluid Power Journal. The winners will also be entered into a drawing for a special gift.