Page 30 - Fluid Power Engineering

Intr oduction to Hydraulic Power Systems 7

this force equals the total weight of the displaced parts (F = mg). The
work done by the forklift is

W = Fy (1.1)
By the end of the time period, Δt, the potential energy of the lifted
body is increased by E, where
E = mgy = Fy (1.2)

where E = Gained potential energy, J
F = Vertically applied force, N
g = Coefficient of gravitational force, m/s 2
m = Mass of lifted body, kg
W = Work, J
y = Vertical displacement, m

This amount of energy (E) is gained during time period Δt. The energy
delivered to the lifted body per unit of time is the delivered power N,
where

N = Fy/Δt = Fv (1.3)

where N = Mechanical power delivered to the load, W
v = Lifting speed, m/s

The load is lifted by a hydraulic cylinder. This cylinder acts on the
lifted body by a force F and drives it with a speed v. Figure 1.10 illus-
trates the action of the hydraulic cylinder. It is a single acting cylinder
which extends by the pressure force and retracts by the body weight.
The pressurized oil flows to the hydraulic cylinder at a flow rate Q
(volumetric flow rate, m /s) and its pressure is p. Neglecting the fric-
3
tion in the cylinder, the pressure force which drives the piston in the
extension direction is given by F = pA .
p

FIGURE 1.10
Lifting a body
vertically by a
hydraulic cylinder.