SERIES FRG-02, FRG-03, FRG-06 & FRG-IO
FLOW CONTROL & OVERLOAD RELIEF VALVES
(REFER TO DESCRIPTIVE BULLETIN 48-36 & PARTS DRAWING 925-S)
1. MODEL NUMBERS.
This maintenance information is for flow control valves with Integral Overload Relief Valve. The complete model
number, which includes pressure range, volume control range and any special feature designation, is stamped on each
valve and should be carefully referred to when using parts drawing or when ordering service parts.
The primary function of these valves is to provide accurate volume control in hydraulic circuits regardless of any
variation in the imposed fluid pressure. n integral overload relief valve is also provided to limit hydraulic pressure in the
system to a predetermined maximum. Both the maximum pressure and the flow control rate are adjustable. Oil from the
system is discharged from the return connection of the valve to the tank whenever pressure builds up to the setting for
which the valve is adjusted. That portion of the volume which is not admitted to the hydraulic circuit is also returned to the
These valves may also be used for an additional purpose in the control circuit. By using the 1/4" pipe thread vent
connection, which is normally plugged and a suitable venting control circuit it is possible to remotely or automatically drop
the system pressure to approximately zero when desired. This venting arrangement is often used for automatically
dropping pressure between working cycles of a machine. The term venting is used because of the fact that pressure on
one side of the otherwise balanced control piston within the valve is dropped by connecting this chamber to tank
(atmospheric pressure through small piping connections and a control valve that, when opened, allows a small volume of
oil to flow or vent continuously into the tank. This, in turn, causes the relief valve to open wide and allow the entire pump
delivery to discharge back to the reservoir at very low pressure until the venting circuit is closed.
Complete installation and operating data is given on installation drawings R-108300 for FRG-02 Series and R-
105340 for FRG-03, 06 and 10 Series. Bulletin 48-36 also contains information on installation and operation procedures.
Be certain that the piping from the valve return connection discharges oil to the tank below the surface of the oil in the
tank. This will do a great deal toward preventing aeration and foaming.
4. VOLUME AND PRESSURE ADJUSTMENT.
The indicating dial on the front of the valve provides a convenient means of adjusting the flow rate. Clockwise
rotation increases the flow rate. Counter-clockwise rotation decreases the flow rate. The dial may be locked in any
position by tightening the socket head locking screw located on the face of the dial.
Adjusting screw (95932) provides a convenient means of regulating the maximum system pressure within the
range of the relief valve portion of this valve. The jam nut (707691 must be loosened before turning the pressure
adjusting screw (95932). Clockwise rotation of the latter increases pressure while counter-clockwise rotation decreases
On all applications the pressure relief valve should be set for the minimum possible system operating pressure
required to perform the necessary service. If it is set for excessive pressure, more power will be used than is necessary
whenever the valve is being held open, as for example when a piston reaches the end of its travel and stops due to some
5. CAUSES OF IMPROPER OPERATION.
Fluctuating pressure or loss of pressure at the relief valve indicates the oil has become aerated or the oil contains
foreign matter. Very small chips or filings left in the tubing or piping, wiping cloth lint, core sand, or any such foreign
substances can cause trouble. Every possible precaution should be taken to prevent any grit or dirt from entering the
system when piping the machine and filling the tank with oil. Also precautions should be taken to protect the tank air vent
opening so that the breathing action in the tank does not draw in dirt or coolant. Abrasive matter will cause pump wear
and will prevent the proper operation of control valves. Dirt cannot get in if these precautions are taken.
Small air bubbles often cause a milky appearance of the oil, and this air will usually cause noisy pump and relief
valve operation, as well as prevent the valve from holding a pressure steadily. Check to make certain that all system
return pipes (including drain connections discharging oil to the tank are well below the oil surface level: also, that no air
leaks are present in the pump intake line (refer to pump maintenance data). Both of these conditions will cause air to be
mixed with the oil. Maintain proper oil level in the tank.