The Pilgrim Nut is a hydraulic device used for mounting and removing a propeller from the tapered tail shaft. It provides an accurate, safe and controlled method of pushing the propeller onto the shaft without hammering.

Construction and Working:

• The propeller is mounted on a tapered tail shaft and secured by a Pilgrim Nut.
• The Pilgrim Nut contains an internal nitrile rubber tube (hydraulic bladder).
• When hydraulic oil is pumped into the rubber tube, it expands and pushes a steel loading ring against the propeller hub.
• This hydraulic force pushes the propeller uniformly onto the taper, producing the required interference fit.
• A dial indicator (clock gauge) is fitted to measure the actual movement (push-up) of the propeller hub relative to the shaft.
• The loading ring should not move outward by more than one-third of its width from the flush position; otherwise, the nitrile rubber tube may rupture.
• Before mounting:
  • The shaft taper and propeller bore are thoroughly cleaned and degreased to obtain predictable friction.
  • (Exception: Cast steel propellers are lightly wiped with an oil-soaked rag as recommended by the manufacturer.)
• Blue marking (Prussian blue) is applied on the shaft taper to check proper contact between the shaft and propeller bore.
• The temperatures of both the shaft and propeller hub are recorded because they affect the required hydraulic pressure. The manufacturer's push-up table/graph (push-up curve) is used to determine the correct final push-up pressure.
• The propeller is pushed up successively in approximately 25 mm stages, with hydraulic pressure applied gradually while continuously monitoring:
  • Hydraulic pressure
  • Propeller movement using the dial indicator
• Once the required push-up distance is achieved, the Pilgrim Nut is finally tightened using a tommy bar.
• The assembly is then secured by a locking plate and locking bolts to prevent loosening during service.

The Pilgrim Nut can also be used as a hydraulic withdrawal tool by reversing its position.

Procedure:

1. Remove the locking plate and bolts, then loosen and unscrew the Pilgrim Nut.
2. Reverse the Pilgrim Nut so that the loading ring faces the withdrawal plate.
3. Fit the withdrawal plate in front of the nut and secure it using studs, as shown in the sketch.
4. Connect the hydraulic pump to the Pilgrim Nut.
5. Apply hydraulic pressure.
6. The expanding nitrile rubber tube pushes the loading ring against the withdrawal plate, producing an equal and opposite force that pulls the propeller hub off the shaft taper.
7. As the taper grip breaks, the propeller moves away from the shaft and can be safely removed.

Safety Precautions:

• Support the propeller using chain blocks, lifting tackles or suitable lifting gear before releasing it.
• Place wooden blocks between the Pilgrim Nut and the propeller, leaving only a gap slightly greater than the push-up distance. This prevents violent movement when the taper suddenly releases.

The Pilgrim Nut method is considered superior to conventional propeller mounting methods because:

1. Accurate and controlled push-up is achieved using hydraulic pressure and dial gauge measurements, ensuring the correct interference fit.
2. No hammering or heavy mechanical force is required, eliminating damage to the propeller hub, shaft taper and bearings.
3. Quick, safe and easily reversible for both installation and removal, reducing maintenance time and minimizing the risk of accidents.
4. Uniform hydraulic loading ensures even distribution of forces, reducing stress concentrations.
5. The manufacturer's push-up curve/graph allows precise control by considering shaft and hub temperature, resulting in consistent and reliable mounting.