1. HYDRAULIC OVERSPEED CONTROL
HOC
HYDRAULIC
OVERSPEED
CONTROL
Parker Hannifin Corp., one of the world’s largest manufacturers of hydraulics for the mobile market, introduces the Hydraulic Overspeed Control system.

2. INTRODUCTION
In the past noise restrictions had forced refuse vehicles to limit engine speed while operating loading functions in residential areas. It was found that system function and fuel efficiency improved by designing the hydraulics to operate in this mid range speed of the truck engine .
A variety of methods were used to limit the maximum flow of the pump in order to take advantage of the improved performance. Most designs centered around the use of a “dry valve”, which was a common component on refuse truck hydraulic systems.
There were many drawbacks. Systems using a dry valve were noisy and caused cavitation damage to the pump. They wasted horsepower and ran hot. Also, systems that used an electronic interface for pump speed control, could easily be defeated by the truck operator.

3. DESIGNED FOR REFUSE TRUCKS
Automatically Controls
Maximum System Speed
The HOC system is unique because it uses pump flow to provide a speed sensitive power shutdown of a refuse truck’s hydraulic system without any electronic interface.
Designed specifically for the refuse truck market, the HOC system has attained the goal of providing the most efficient, reliable, and tamper proof system on the market.

4. Pressure Compensated Flow Control VA35 Valve Inlet Unloader
SYSTEM COMPONENTS
Gear Pump
Pressure Compensated Flow Control
VA35 Valve Inlet Unloader
Pump Inlet Porting Block
The standard HOC system includes a gear pump, a pressure compensated flow control, an unloader inlet for the VA 35 control valve and a pump inlet port block.

5. FLOW SETTINGS 50 gpm 25 gpm 55 gpm 30 gpm 60 gpm 35 gpm 65 gpm 40 gpm
The unload setting of HOC system is changeable in 5 gallon per minute increments through a broad range of flows. This allows the HOC system to be used on refuse equipment of various sizes.

6. HOC INSTALLATION
Following are a schematic and an illustration depicting system installation.

7. HOC SCHEMATIC HOC UNLOADER INLET (VA 35) BYPASS LINE TO SIGNAL
PUMP INLET
SIGNAL
LINE
TANK
OPEN
CENTER
HOC FLOW
CONTROL
PARALLEL
PASSAGE
The HOC flow control is bolted to the 1 1/4” split flange outlet port of the pump. Pump flow passes through it and goes to the valve inlet. The HOC flow control has 2 additional ports, a 3/4” O.D. tube bypass which is connected to the pump inlet line either through the porting block, or through a port on the suction side of the port end cover. The second port is a 3/8” O.D. tube signal port which is plumbed to the signal port in the valve unloader inlet.
The unloader inlet is equipped with a tank return port. The use of this port for the tank return line is recommended. It keeps system pressure drop to a minimum because the unloaded pump flow can go back to the tank without passing through the rest of the valve.
TANK RETURN LINE

8. HOC SYSTEM HOC FLOW CONTROL BYPASS LINE TO PUMP INLET TANK RETURN LINE
OUTLET
HOC UNLOADER
INLET (VA 35)
SIGNAL
LINE
This illustration shows the plumbing connections previously described. In addition it displays through color the HOC system in a loaded state with pump flow available to do work.
Red indicates system pressure while blue indicates tank return or low pressure.

9. RETROFIT OPTIONS The HOC system can be installed on existing vehicles.
If a Commercial VA 35 is employed as original equipment, a simple substitution of the standard inlet with the unloader inlet and a replacement of studs completes the valve conversion.
If the original equipment pump is equipped with a dry valve and/or a flow control, these should be removed. They are replaced by the HOC flow control on the outlet port and a porting block on the inlet port of the pump.
The inlet porting block is a spacer that goes between the 2”split flange inlet fitting and the inlet port. The block has a 3/4” O. D. tube port on its side which is plumbed to the HOC flow control bypass port.

10. RETROFIT IN-LINE VERSION
VA35 Inlet Unloader
Power Beyond Outlet
Bolt Kit
Hoc Flow Control
Pump Inlet Porting Block
If a valve of a different manufacturer is used as original equipment, the retrofit inline version of the HOC system is used.
The inline version consists of the VA 35 inlet unloader, a power beyond outlet and a bolt kit which are assembled to make the in-line component. This item is placed in-line between the pump and the existing valve.
The pump is converted as previously described.

11. HOC IN-LINE SCHEMATIC BYPASS POWER BEYOND LINE TO HOC UNLOADER OUTLET
PUMP INLET
POWER BEYOND
OUTLET
HOC UNLOADER
INLET (VA 35)
SIGNAL
LINE
TO CONTROL
VALVE
HOC FLOW
CONTROL
This schematic shows the position of the in-line unloader component between the pump and the control valve.
NOTE: A separate line must be connected from the low pressure port on the inline unloader back to the tank.
TANK RETURN LINE

12. SYSTEM FUNCTION
The next four illustrations are used to describe system function.

13. BYPASS LINE TO PUMP INLET HOC FLOW CONTROL PUMP OUTLET TO HOC UNLOADER
INLET (VA 35)
REPLACEABLE ORIFICE
This is a cross sectional view of the HOC flow control showing pump flow at less than the flow control setting. At this flow the hydraulic system is operational.
The replaceable orifice is positioned in a spring loaded spool. The size of the orifice determines the flow setting of the system. The pump flow takes a pressure drop as it passes through the orifice. As long as the pressure drop across the orifice does not reach 50 pounds per square inch the spool holds position and the hydraulic system is loaded.
Again, red indicates system or high pressure and blue signifies low or tank pressure.
SIGNAL LINE
TO INLET UNLOADER

14. PUMP FLOW FROM
FLOW CONTROL
TANK RETURN LINE
SIGNAL
LINE FROM
FLOW CONTROL
This illustration shows the pump flow moving into the valve inlet. From this point it goes to the valve work sections where it can be routed to the system actuators in order to do work.
HOC UNLOADER
INLET (VA 35)

15. BYPASS LINE TO PUMP INLET HOC FLOW CONTROL PUMP OUTLET TO HOC UNLOADER
INLET (VA 35)
REPLACEABLE ORIFICE
When the pump flow increases enough to cause a 50 PSI pressure drop through the orifice the spring force is overcome allowing the spool to move. The spool movement opens the bypass line to the portion of the pump flow which is in excess of the flow control setting. This flow is routed back to the inlet of the pump.
At the same time the bypass line is opened, a small portion of the pump flow is directed to the signal line and is sent downstream to the inlet unloader.
SIGNAL LINE
TO INLET UNLOADER

16. PUMP FLOW FROM
FLOW CONTROL
TANK RETURN LINE
SIGNAL
LINE FROM
FLOW CONTROL
The pump flow in the signal line is forced against one end of the inlet unloader spool. This action pushes the unloader spool to one side, opening a passage for the pump flow to return to the tank, unloading the hydraulic system.
To restore power to the hydraulic system the operator must lower the engine speed 300 to 400 RPM below the HOC speed setting.
The HOC unload and load actions are very smooth because of a dampener built into the spool. This is in contrast to other speed control systems on the market. These systems tend to shift abruptly, causing system shocks and damage to components.
HOC UNLOADER
INLET (VA 35)

17. HOC UNLOADER PRESSURE DROP CURVE
The chart graphically illustrates the very low pressure drop at which the unloaded pump flow is returned to tank through VA 35 inlet unloader. This low pressure drop translates into a very small horse power draw on the truck engine when the hydraulic system is shut down.

18. HOC SHEMATIC WITH ROADING VALVE
BYPASS
LINE TO
PUMP INLET
SIGNAL
LINE
HOC UNLOADER
INLET (VA 35)
PARALLEL
PASSAGE
OPEN
CENTER
TANK
ROADING
VALVE
HOC FLOW
CONTROL
TANK RETURN LINE
This schematic illustrates a component that is a recommended addition to the HOC system. It’s called a “roading” valve and is a simple two way valve plumbed between the pump outlet line and the HOC signal line. It can be shifted manually, with air, or with a solenoid.
With the valve shifted to the blocked position the system can function normally allowing the operator to pack the load between stops.
With the valve shifted to the open position the hydraulic system is totally disabled. This position is used during long haul situations such as, to and from the pick up route, and to and from the dump site.

19. HOC SYSTEM WITH ROADING VALVE
HOC FLOW
CONTROL
BYPASS
LINE TO
PUMP INLET
TANK RETURN LINE
PUMP
OUTLET
HOC UNLOADER
INLET (VA 35)
SIGNAL
LINE
ROADING
VALVE
This illustration shows the roading valve in the open position. In this position a portion of the pump flow is always sent through the signal line, keeping the unloader spool in the unload position and the hydraulic system deactivated. The color blue signifies low pressure.

20. BENEFITS Automatically Prevents System Overspeed Saves Horsepower
Tamper Proof
Eliminates Possible Pump Cavitation
Full Pump Flow Circulation
Faster Winter Warm Up
Runs Cooler
Improves System Filtration
System Performance Not Affected By Normal Pump Wear
BENEFITS:
OVERSPEED CONTROL
The HOC prevents hydraulic system overspeed. Overspeeding causes excessive noise and possible machine damage.
SAVES HORSEPOWER
The HOC system does not bypass excess high pressure pump flow to tank during system operation, eliminating horse power waste and heat generation.
TAMPER PROOF
Because the HOC system is totally hydraulic, it is very tamper resistant.
ELIMINATES CAVITATION
Because the HOC flow control reroutes excess pump flow back to the pump inlet, the maximum net draw of the pump from tank is equivalent to the flow setting of the HOC flow control, and not the higher flow of the pump at top engine speed. This greatly reduces the possibility of cavitation.
FULL PUMP FLOW RECIRCULATION
Systems using a dry valve only recirculate about 1 gallon per minute through the system in the “dry” mode. In contrast, the HOC system recirculates full pump flow allowing it to warm up faster, run cooler and filter the system more effectively.
SYSTEM PERFORMANCE NOT AFFECTED BY NORMAL PUMP WEAR
Because the HOC senses pump flow to shut off the system, normal pump wear will be compensated for with a slight increase in engine speed and not a decrease in overall system performance.

21. Parker Hannifin Corporation, one of the world’s largest manufacturers of hydraulics for the mobile market, has brought you this training program on the Hydraulic Overspeed Control system. If you have any further questions on this or any of our other hydraulic products, please contact your Parker Sales Representative or your nearest Authorized Distributor.