JLG 340AJ Service Manual - Manual (Page 193)

SECTION 5 - HYDRAULICS
3121259 – JLG Lift – 5-35
The System Circuit
THE BASIC CLOSED CIRCUIT
Hydraulic lines connect the main ports of the pump to the
main ports of the motor. Fluid flows in either direction from
the pump to the motor and back. Either of the hydraulic lines
can be under high pressure. In pumping mode the position of
the pump swashplate determines which line is high pressure
as well as the direction of fluid flow.
CASE DRAIN AND HEAT EXCHANGER
The pump and motor require case drain lines to remove hot
fluid from the system.The pump and motor drain from the top-
most port to ensure the cases remain full of fluid.The motor
case drain can connect to the lower drain port on the pump
housing or it can tee into the case drain line upstream of the
heat exchanger. A heat exchanger with bypass valve cools the
case drain fluid before it returns to the reservoir.
High Pressure Relief Valve (HPRV) and Charge
Check
All H1 pumps have a combination high pressure relief and
charge check valve. The high-pressure relief function is a dissi-
pative (heat generating) pressure control valve for the pur-
pose of limiting excessive system pressures. The charge check
function replenishes the low-pressure side of the working loop
with charge oil. Each side of the transmission loop has a dedi-
cated non-adjustable, factory-set HPRV valve. When system
pressure exceeds the factory setting, oil is passed from the
high pressure system loop into the charge gallery, and into the
low pressure system loop via the charge check.
The pump may have different pressure settings at each system
port. When an HPRV valve is used in conjunction with a pres-
sure limiter, the HPRV valve is always factory set above the set-
ting of the pressure limiter. The system pressure shown in the
order code for pumps with only HPRV is the HPRV setting.The
system pressure shown in the order code for pumps with pres-
sure limiter and HPRV, is the pressure limiter setting.
NOTE: HPRVs are set at low flow condition. Any application or
operating condition which leads to elevated HPRV flow will
cause a pressure rise with flow above the valve setting.
Figure 5-25. High Pressure Relief and Charge Check Valve
with Bypass Valve in charging mode
Figure 5-26. High Pressure Relief and Charge Check Valve
with Bypass Valve in relief mode