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Flexible couplings are simple and relatively inexpensive devices which aren’t difficult to install or maintain, yet many of which fail prematurely. These failures of flexible coupling also happen despite the devices being specially designed to tolerate slight misalignment and to cope with the tough operating conditions including temperature extremes, humidity, loads and so on. We can even check and maintain the flexible coupling on one’s own.
What are the common failures of flexible coupling?
How to check alignment of flexible coupling?
How to maintain flexible coupling?
With an increase in misalignment, surface contact area decreases, resulting in increased loading on flexible coupling components. This results in increased friction. This increased friction imposes a corresponding pre-load to the two shafts that are connected by the flexible coupling. This prevents the flexible coupling from moving axially, commonly called flexible coupling lock-up to compensate for misalignment.
Fretting occurs when two surfaces move in relation to one another. The loading and heat generated by friction causes metal to be removed, to be transferred, or to crack. Fretting is actually fatigue of metal surfaces. In gear and grid couplings flex members slide in relation to each other.
Wear is the major problem with lubricated flexible couplings. Most cases of wear can be traced to the lack of lubrication. Dirt in the lubricant can cause disaster. Overfilling with grease in a gear or grid coupling is a very common mistake and should be avoided because heat can build up in the flexible coupling or cause a hydraulic lock. After packing, check that the flexible coupling will move back and forth to ensure that the flexible coupling is not grease-bound.
In a disc flexible coupling, failures result from fatigue due to excessive flexure from greater than designed misalignment. If a disc coupling is installed such that the distance between two shafts is too much or less as compared to designed distance, it leads to axial misalignment. A disc coupling can be inspected for damage without its disassembly if discs are visible. It can be checked with a strobe light while it is in operation. Under a strobe light the discs can be seen flexing and moving and a damaged disc would show up.
The necessary tools used for checking the alignment of a flexible coupling are a straightedge and a taper gauge or a set of feeler gauges, or by use of dial indicators.
A rough check for angular alignment is made by inserting the taper gauge or feelers between the coupling faces at 90-degree intervals. Check for angular and parallel alignment by this method can only be done if the face and outside diameters of the coupling halves are square and concentric with the flexible coupling bores. A rough check for parallel alignment is made by placing a straightedge across both flexible coupling rims at the top, bottom and at both sides. After rough alignment, fasten the indicator to the pump half of the coupling, with the indicator button resting on the other half coupling periphery. Set the dial to zero, and mark the flexible coupling half beside where the button rests. Rotate both shafts by the same amount, for example, all readings on the dial must be made with the button beside the mark. The dial readings will indicate whether the driver has to be raised, lowered or moved to either side.
After each adjustment, recheck both parallel and angular alignments. Accurate alignment of shaft centers can be obtained with the dial indicator method—even where faces or outside diameters of the flexible coupling halves are not square or concentric with the bores—provided all measurements for angular alignment are made between the same two points on the outside diameters.
General maintenance inspection of flexible couplings involves thorough visual inspection and bolt torque inspection, and then complete flexible coupling disassembly and visual inspection. When inspecting a flexible coupling, it is normal to take an angular set after some time in operation. The outer surface of the flexible coupling may commonly exhibit minor hardening and light cracking. The flexible coupling manufacturer should supply the proper guidelines for measuring and evaluating the maximum permissible angle of set, and the crack measurements that would dictate replacement.
If equipment is driven by a high compression engine or if an electric motor system includes a variable frequency drive coupled to a reciprocating pump or compressor, it is a good idea to consult a qualified torsional vibration consultant to assure that the flexible coupling is properly selected to protect the system from harmful resonances. If a rubber coupling has melted from the inside, the problem must be addressed with qualified experts to ensure that the equipment can safely be restored to nominal operating conditions.
Check the condition of the rubber bushes periodically every 6 months for wear. If the outside diameter of the bushes is worn down more than 5% compared to the pin bore diameter, then the pin and bush assembles should be changed.
Learn more something about flexible couplings can know the failures of flexible couplings and how to maintain flexible couplings. If you want to buy good shaft couplings with reasonable price, FOSHAN HOUZHE TRANSMISSION EQUIPMENT CO., LTD. is your best choice. We have all types of different flexible couplings and if you have any questions, you can contact us or search us online.