Pump coupling and bell housing of an injection molding machine

The pump coupling and bell housing join the hydraulic pump of an injection molding machine to its electric motor. The bell housing is a cast adapter that bolts to the motor flange on one side and to the pump flange on the other, holding the two shafts in line. The flexible jaw coupling inside it connects the shafts, carries the torque, and absorbs vibration and small misalignment. On the drawing, it is the cone-shaped housing between the motor and the pump.

How the coupling and bell housing work

Bell housing

Each end of the bell housing has a machined spigot (register) that fits the centering diameter of the motor flange and the pilot of the pump flange. Those two fits, and the flatness of the flange faces, put the pump shaft on the same axis as the motor shaft. The bell housing also carries the weight of the pump on many machines and keeps fingers away from the rotating coupling. The bell housings we supply are cast iron or ductile iron.

Jaw coupling

The coupling has three parts:

• a hub on the motor shaft, fixed with a parallel key and a set screw;
• a hub on the pump shaft, fixed with a key or, on many pumps, an SAE spline;
• an elastomer insert (the spider, often called a plum insert for its shape) that sits between the jaws of the two hubs.

The jaws push on the insert, which carries the torque in compression, damps shocks from pressure changes and allows a small amount of angular, parallel and axial misalignment. It is not meant to correct a misaligned assembly: that is the bell housing's job. The same type of plum coupling is used between servo motors and pumps on servo pump systems.

See the pump coupling and bell housing we supply, listed for Haitian MA and Jupiter series machines.

Signs of wear or failure

• A knock when the motor starts or stops, or when the pressure changes suddenly.
• Rubber dust or pieces of the insert inside or under the bell housing.
• More vibration and noise at the bell housing than before.
• Repeated pump shaft seal leaks or early pump or motor bearing failures, which point to misalignment or axial load on the shaft.
• A hub loose on its shaft: worn key or keyway, worn spline, or rust from fretting.
• Cracks in the bell housing or loose flange bolts.
• The motor runs but the pump delivers nothing: the insert has broken up or a hub has slipped.

Why couplings and bell housings wear

• Ageing of the insert from heat, oil mist and the number of starts and pressure changes.
• Misalignment, from dirt or burrs on the flange faces, worn spigot fits or a non-original bell housing.
• Wrong axial position of the hubs: if the gap is too small, the jaws or the insert push on the pump shaft and load its bearing; if it is too large, the jaws do not engage fully.
• Shock loads from pressure peaks and frequent starts.

Inspecting and replacing the coupling

1. Switch off and lock out the motor. On a servo motor, also wait for the drive to discharge.
2. Close the suction valve if the pump will be removed, relieve the pressure in the circuit and support the pump.
3. If the bell housing has an inspection opening, look at the insert through it. Otherwise remove the pump from the bell housing.
4. Inspect the insert for cracks, hardening and missing pieces, the hub jaws for wear, and the keys, keyways and splines for play.
5. Fit the hubs with their keys, set the gap between the hubs to the coupling maker's value, and secure the set screws.
6. Clean the flange faces and spigots, refit the pump, and tighten the bolts evenly in a cross pattern to the torque in the machine manual.
7. Turn the shaft by hand if you can, then run the pump and listen for knocking or vibration.

Fit a new insert every time the pump or motor is replaced. When fitting a new pump, see hydraulic pump types for the start-up steps.

What to measure for a replacement quote

Because the bell housing and coupling must match both the motor and the pump, send:

1. Motor side: the motor model or frame size, the flange centering diameter, bolt circle and number of holes, and the shaft diameter, length and key size.
2. Pump side: the pump model code, the flange type (2-bolt or 4-bolt) and pilot diameter, and the shaft: key or spline, diameter, and for a spline the number of teeth.
3. Assembly: the overall length of the bell housing, or the distance between the motor flange face and the pump flange face.
4. Coupling: any size marked on the hubs, and photos of the hubs and the insert.

Add the machine model and serial number, then request a quote. For the complete pump-motor assembly, see the hydraulic power unit. The hydraulic system guide shows the other parts of the circuit.