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Woodward Herringbone Gear Pump (XX Pump)
XX Herringbone Gear Pump The Herringbone Gear Pump, also known as the pump

Woodward Herringbone Gear Pump (XX Pump)

XX Herringbone Gear Pump
The Herringbone Gear Pump, also known as the Double-X pump, consists of two matched herringbone gears in a pump assembly that includes unloader, check and relief valves. The drive gear is turned by a drive shaft, which is also the shaft of the electric drive motor.

The clearances between the gear teeth as they mesh and between the teeth and the pump housing are very small. The inlet port is connected to the suction line from the sump, and the outlet port is connected to the pressure line which feeds the accumulator and governor distributing valve.

As the teeth pass the inlet port, liquid is trapped between the teeth and the housing. This liquid is carried around the housing to the outlet port. As the teeth mesh again, the liquid between the teeth is pushed into the outlet port. This action produces a positive flow of liquid into the system.

The Herringbone Gear Pump is better than a standard spur gear pump because each set of teeth begins its fluid discharge phase before the previous set of teeth has completed its discharge phase. This overlap (and the relatively larger space at the center of the gears) tend to minimize pulsations and give a steadier flow than the spur gear pump.

XX Herringbone Gear Pump Cutaway

The unloader valve (or bypass valve) acts to route the oil back to the sump with minimal backpressure. This allows the motor to start and stop with minimal load. After the motor has started, the bypass closes and forces the oil into the main pressure header. When the pumping cycle is complete, the bypass opens and allows the motor to be stopped without load.

Recommended Maintenance

Severe damage can occur if regular maintenance is not performed. Woodward recommends that the bearings be changed every 5 to 7 years.

XX Herringbone Disassembly for Refurbishing
There are 5 bearings (four gear bearings and a motor bearing) and a number of seals that should be replaced. The bearings must be maximum capacity type. Maximum capacity bearings have more ball bearings to take the high loads and vibrations associated with pumping. If the correct bearings are not used, catastrophic failure can occur within 3 to 4 years.

Customers have resisted the 5 to 7 year recommended time frame because the pumps have been running for 15 plus years without problems. These pumps are running on borrowed time.

Minor problems with the gears or case can be repaired with minimal (if any) loss of pumping ability. Major failures will require extensive rework or total replacement.

Because of the special nature of these pumps, new, drop-in replacements are not available. Engineered replacements can cost in excess of $75,000 which makes routine maintenance a bargain.

Gear Face Damage

Gear Hot Spot

Broken Retainer Bolts

Factory Repair Service

Our Training and Repair Facility offers for XX Herringbone Gear Pumps.

Factory repairs save you money compared to a service call and allow us to perform additional services that are difficult or impossible to do in the field.

XX Herringbone Gear Pumps are very robust and will last many more years with routine maintenance. In addition to basic pump overhaul services, we offer advanced repair services and have been able to repair some severely damaged pumps that customers thought were beyond saving.

Basic pump overhaul includes:

  • Disassembly, cleaning and inspection of pump and unloader
  • Replacement of pump (4) and motor (1) bearings
  • Inspection and dressing of herringbone gears
  • Installation of new gaskets, seals and O-rings
  • Reassembly and adjustment of end clearance to OEM specs

Advanced repair services include:

  • Side grinding the drive and idler gears to maintain proper centering of the gears when meshed
  • Magnaflux inspection of drive and idler gear journals for cracks
  • Resurfacing of inboard and outboard bearing plates to remove any score marks

Two overhauled 40HP XX Pumps ready to ship

See our Factory Repair Bulletin below for more information.

Bulletin FRS - Factory Repair Services.pdf
Our Training and Repair Facility offers repair services for major governor auxiliary systems like XX Herringbone Gear Pumps and Permanent Magnet Generators. Factory repairs save you money compared to a service call and allow us to perform additional services that are difficult or impossible to do in the field.

XX Pump Parts

Woodward produced various sizes of XX Pumps. While the reference numbers are the same, the parts required for XX Pump maintenance are specific for each pump size and motor frame. Please for assistance in determining which part numbers are appropriate for your unit.

You will need the name plate data from the motor and the number of bolts on the top flanges (4,6, or 8).

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