The function of a flexible coupling is to transmit torque from the driver to the driven machine, while making allowances for minor shaft misalignment and shaft end position changes between the two machines. The design of the coupling should provide for transmission of the required torque at the required speed, with a minimum of extraneous forces and perturbations being exerted on either the driver or driven shaft. Shaft misalignment exists when the centerlines of two shafts that are joined by a coupling do not coincide.
Each coupling type has a maximum tolerance of misalignment and axial position change. Regardless of coupling type, misalignment tolerance is stated in degrees and is usually ¼ degree. Axial position change tolerance varies with coupling type. Gear type couplings have a large axial position change tolerance compared to flexible element types. They usually include two separate gear mesh units, each of which consists of an external gear that fits closely into an internal gear.
The internal gear can either be part of the coupling hub assemblies, or be mounted on each end of the coupling’s spacer assembly. If the internal gears are hub mounted, then the external gears are spacer mounted and vice-versa.
Grease pack couplings are normally designed with hub mounted external gears, and the internal gears are part of a sleeve type spacer which serves as a retainer for the grease lubrication. The flange joint of the sleeve is either precision ground to avoid lubrication leaks, or has a gasket between the two flange faces. The sleeve ends are fitted with O-type ring seals to keep dust out and lubrication in.
In recent years, flexible element couplings have been used almost exclusively. They are the most compact of all the coupling designs for any amount of torque transmission. For this reason, they also have the least overhung weight. In addition, the gear coupling can adapt more readily to requirements for axial growth of the driver and driven shafts. Axial position change tolerances are on the order of half an inch or greater.
Inevitable build up of sludge
There is a common disadvantage in all gear type flexible couplings. Any gear mesh has a break-away friction factor in the axial direction. This is caused by the high contact force between the two sets of gear teeth. The result is that the forces imposed on the driver and driven shafts are not totally predictable, and are sometimes higher than desired due to the quality of the tooth machine surfaces, and the inevitable build up of sludge or foreign material in the tooth mesh during extended service.
These forces are detrimental to the ability of the coupling to make the required corrections for the misalignment but, more importantly, can have a disastrous effect on the ability of the coupling to correct for thermal or thrust force changes between the driver and the driven machines.
Both coupling manufacturers and users have long been aware of this problem, and have used many methods to minimize the effect. But these steps have been only partially successful and the problems still exist in many applications. Coupling manufacturers are asked to quote the design break-away friction factor of their coupling as built and shipped from the factory.
Machinery trained designers then use this figure to calculate the maximum axial force that the coupling would be expected to exert on the connected shafts. From this information, the designers can decide if the thrust bearings adjacent to the couplings are adequate to handle the axial loads within the machine plus the possible load from the coupling resistance to any external forces.
How to prevent damage
Replacement of gear couplings can be accomplished successfully as long as the following are considered and confirmed to be designed correctly with the new dry coupling:
Tortional natural frequencies
Lateral natural frequencies (critical speeds) and rotor response
Coupling guard internal clearance to coupling hub flange OD
Many existing critical machine trains require a yearly shutdown to clean gear coupling teeth after high vibration shutdowns or bearing failures necessitate shutdown. The time for shutdown can exceed three days. Present daily revenues for critical (unspared), compressor trains can exceed $1MM/day.
This best practice has been used since the 1990s to remove the necessity of a yearly gear coupling PMs, and to prevent bearing damage resulting from coupling lock-up. The modification caused can be easily justified if yearly shutdowns are required for gear coupling cleaning.