Mechanical design. Propeller type VBS

MAN B&W Controllable pitch propeller. Mechanical design. Propeller type VBS

Propeller type VBS
In the propeller equipment type VBS the hydraulic servo motor for pitch setting is an integral part of the NiAl-bronze propeller hub. The design of the four-bladed CP propeller is shown in fig 4. The propeller hub is bolted to the flanged end of the tailshaft, which is bored to accommodate the servo oil and pitch feed-back rod.

Fig. 4. Propeller hub type VBS
The servo piston which is bolted to the pitch control head, forms the hydraulic servo motor together with the propeller cap.
The high pressure servo oil system at the aft end of the hub is completely isolated from the pitch regulating mechanism and thus also from the blade flanges, which means that the blade sealings only are subjected to gravitation oil pressure.
By using a large servo piston diameter and balanced blade shapes, the oil pressure and reaction forces are minimized in the servo oil pressure space.
Blade sealing rings are placed between blade foot and blade flange.

 A compressed O-ring presses a PTFE (Teflon) slide ring against the blade foot. This design ensures maximum reliability and sealing, also under extreme abrasive wear conditions. For servicing and inspection of the internal parts the hub remains attached to the shaft flange during disassembly thereby reducing time and need for heavy lifting equipment. Optionally an intermediate flange can be inserted, by which underwater replacement of propeller blades is possible.
A hydraulic tube, located inside the shafting, is connected to the piston. With hydraulic oil flowing through the tube, oil is given access into the after section of the propeller cap, displacing the servo piston forward, into an ahead pitch position. The displaced hydraulic oil from forward of the piston is returned via the annular space between the tube and shaft bore to the oil tank.
Reverting the flow directions will move the propeller in astern position.

Oil distribution 
The oil distribution to the hydraulic servo motor can be carried out in two different ways. For two-stroke plants the oil distributor is located at the shaft-line. For four-stroke plants with reduction gearboxes the oil distributor can either be placed at the front end of the gearbox or in the shaft-line.

Shaft mounted OD-ring
The shaft mounted unit consists of a coupling flange with OD-ring and pitch feed-back. Via the oil distribution ring, high pressure oil is connected to one side of the servo piston and the other side to the drain. The piston is hereby moved setting the desired propeller pitch. A feed-back ring is connected to the hydraulic pipe by slots in the coupling flange, fig. 5. The feed-back ring actuates a displacement transmitter in the electrical pitch feed-back box which measures the actual pitch.
The inner surface of the oil distribution ring is lined with white-metal. The ring itself is split for exchange without withdrawal of the shaft or dismounting of the hydraulic coupling flange.

Fig. 5. Coupling flange with OD-ring and pitch feed-back ring
The sealing consists of mechanical throw-off rings which ensures that no wear takes place and that sealing rings of V-lip-ring type or similar are unnecessary.
The oil distributor ring is prevented from rotating by a securing device comprising a steel ball located in the ring. Acceptable installation tolerances are ensured and movement of the propeller shaft remains possible.
In the event of failing oil pressure or fault in the remote control system, special studs can be screwed into the oil distribution ring hereby making manual oil flow control possible. A hold circuit will then maintain the chosen pitch.

Gearbox mounted OD-box
For propulsion plants with reduction gearboxes the propeller equipment can be supplied with an oil distribution box mounted at the front-end of the gearbox. The OD-box incorporates an electric pitch feed-back.

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