TechArchive

Original article date: July 1998

The need for frictional hub connections arose through situations where other shaft hub attachment methods such as keys splines and shrink fits have failed or presented problems. Frictional shaft-hub connections are one answer argue Martin McDonagh and Tony Thornton of Ringflex Drive Systems.

The keyed connection though widely used can cause serious problems. High stress concentrations are created in both the shaft and hub due to the notch effect of the keyway leading to low fatigue strength premature fatigue fracture and possibly catastrophic shaft failure. In addition the material of the shaft is not fully utilised because its diameter has to be significantly larger in order to compensate for the weaknesses introduced by the keyway.

These problems are made even worse when shock and reversal loads are encountered leading to crushing of the key steadily increasing key to keyway clearance and hence eventual failure of the connection. The main drawbacks encountered with keyways are:

• No possibility of angular adjustment of the hub once fitted without some ingenious and often expensive engineering solution such as slotted bolt holes.
• Undesirable backlash can be present. This can lead to fretting corrosion making the connection difficult to disassemble and if left undiscovered may cause premature failure.
• Fitting of keys in keyways is a skilled job difficult and time consuming especially when there are two or more keys in the same hub.
• Close machining tolerances and fine surface finishes are required. The angular position of keyways on the same shaft when hubs need to be timed to one another also requires very accurate machining processes.
• Keyway milling and slotting often cannot be done in house sub-contracting this work can mean loss of control over quality and production schedules and incur higher costs.
• Extra parts are often required to keep the keyed hubs axially positioned flanges nuts and bolts etc.

Splines are able to transmit very high torques. They are however still basically a multiple keyed connection and have all the weaknesses and disadvantages inherent with it. Requiring special tooling such as broaches they are also very expensive to manufacture.

In this type of connection torque transmission is related to the compressive force – the surface pressure between the hub bore and shaft and the coefficient of friction between the surfaces in contact. Shrink fit connections perform very well under dynamic and reversing loads. They have high fatigue strength due to the absence of any stress raising notches (keyways) in the shaft. However they still present a number of problems:

• Very close (ground) machining tolerances are required. The tapers for oil injection type connections also require very close machining tolerances and very fine surface finishes to ensure correct matching of the tapers.
• Assembly and disassembly is difficult involving either special and expensive tooling (for oil injection connections) or high forces and temperature differentials (using liquid nitrogen heat etc) for conventional shrink fits.
• The high forces or high temperatures usually required for disassembly can result in serious damage to hub and or shaft.
• Because of removal difficulties timing of components to each other is virtually impossible.
• Complicated and involved calculations are required.

By comparison to the traditional methods of hub to shaft connection the method that induces the least stress in the shaft is the interference/heat shrink connection. Its performance with regard to fatigue strength under alternating torsional stresses and in reversing drives is well known. Therefore if a hub to shaft connection could give these advantages without the shortcomings it would be the optimum solution.

The Ringfeder frictional locking connection is a mechanical shrink fit system within which the use of tapers and clamping screws or bolts makes it possible for the friction locking assembly locking element shrink disc or cone clamping device to generate the hub to shaft interface pressures required to transmit torque axial forces radial and bending loads often in combination.

• Easy calculations using simple formulae and catalogue data.
• No stress raising keyways or splines to weaken the shaft cross-section resulting in a very low notch factor and allowing the use of the optimum shaft diameter.
• High fatigue strength even under alternating and reversal loads
• Easy and simple fitting and removal using a torque wrench.
• Precise angular and axial adjustment easily achieved by slackening the locking screws or bolts moving the hub or shaft to the new position and then re-tightening.
• Simplified design and manufacture. Friction locking connections use parallel hub bores and shafts only requiring turned surface finishes and are capable of bridging large tolerances.
• Adjustable overload protection is possible by adjusting the screw tightening torque of the friction locking connection. This is ideal for such applications as flywheels etc.
• Wear and backlash are eliminated.
• Large cost reductions due to reduced machining and fitting times and operations.

• Ringflex Drive Systems
• Tony Thornton
• 0161 474 0464

July 1998