Both the R&D Marine Flexible Shaft Coupling and Globe Rubber Drivesaver are designed to help isolate vibrations between the shaft coupling and transmission output flange and to absorb shock loads due to hard gear changes or propeller impact. Instead of the two couplings being rigidly bolted together, an element is fit between the two couplings. The R&D Marine Flexible Shaft Coupling offers several design advantages over the Drivesaver.
R&D Marine Flexible Shaft Coupling
Globe Rubber Drivesaver
A failsafe design is incorporated in the R&D Flexible Shaft Coupling that prevents the two couplings from pulling apart in the event of an impact or element failure and the Drivesaver does not. This failsafe design is accomplished by using two steel straps and thru-bolts going into both the shaft coupling and transmission output flange. The aft steel strap is bolted to the transmission output flange and the front strap is bolted the shaft coupling. In the event of an impact the aft steel strap (bolted to the transmission) will catch the front steel strap (bolted to the shaft coupling) and prevent the drivetrain from pulling apart. The R&D element will absorb the shock load (reducing or eliminating damage to the transmission) and will maintain contact between the two couplings. The steel straps also keep the R&D Flex-Coupling under a compression load in both forward AND reverse operation (the aft strap bolted to the transmission acts as a backing plate).
The Globe Rubber Drivesaver thru-bolts to the transmission output flange and studs are threaded directly into the element from the shaft coupling. In the event of an impact (or excessive reverse thrust) the studs in the Drivesaver can pull out of the element resulting in the drivetrain pulling apart, loss of drive and possibly the propeller locking up against the rudder.
Machined vs. Molded:
The Drivesaver is molded while the R&D Flexible Shaft Coupling is machined. Both designs suggested that you first set your alignment by drawing the two couplings up to each other and then add the flex-coupling. The machined part will have MUCH tighter tolerance than a molded part.
The R&D Flexible Shaft Coupling allows for inspection of alignment after the element is bolted in place. There is one bolt head that is machined and painted red. After the R&D Flexible Shaft Coupling is bolted into place, a feeler gage can be slipped between the red bolt head and the coupling. This process is repeated with the same bolt four times at 90-degree intervals by rotating the shaft. If the gap is within .010” (0.25mm), the engine is accurately aligned.
Sizing the element for the application:
There are more sizes to choose from with the R&D Flexible Shaft Coupling based on the HP/100 RPM at the shaft. To properly isolate vibrations and protect the drivetrain, the element should be sized according to the HP/100 RPM at the shaft. To calculate the HP/100 RPM we use the formula:
Maximum Engine HP / Maximum Engine RPM x Reduction Gear x 100 = HP/100 RPM at the shaft