A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.
The primary purpose of couplings is to join two pieces of rotating equipment w- hile permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.
An Oldham coupler is a method to transfer torque between two parallel but not collinear shafts. It has three discs, one coupled to the input, one coupled to the output, and a middle disc that is joined to the first two by tongue and groove. The tongue and groove on one side is perpendicular to the tongue and groove on the other. Often springs are used to reduce backlash of the mechanism. The coupler is much more compact than, for example, two universal joints.
Oldham couplings are three piece couplings comprised of two hubs and a center member. The center disk, which is available in a choice of acetal for high torsional stiffness or nylon for vibration and shock absorption, is the torque transmitting element. Torque transmission is accomplished by mating slots in the center disk, located on opposite sides of the disk and oriented 90 degrees apart, with the drive tenons on the hubs. The slots of the disk fit on the tenons of the hub with a slight press fit. This press fit allows the coupling (with an acetal disk) to operate with zero backlash. While over time the sliding of the disk over the tenons will create wear and the coupling will cease to be zero backlashes, the disk can be easily replaced and the coupling’s original performance restored. In
Operation, the center disk slides on the tenon of the hub to accommodate misalignment. This design is particularly well suited for handling relatively large amounts of parallel misalignment. The disks are also electrically isolating and can act as a mechanical fuse. When the plastic insert fails, it breaks cleanly and does not allow any transmission of power, preventing other damage from occurring to more expensive machinery components.
PARTS OF OLDHAM’S COUPLING
It is the part which is connected to the ends of the shafts. It is a round circular shaped disk. The disk consists of groves in the center of them which is plugged into the projections of central disk.
It is the coupling part of two hubs. It consist of two projected bars on the both sides the disk which are perpendicularly plugged into the hubs. The center disk is press-fitted to eliminate backlash and also designed to act as a mechanical fuse. The disc slides to accommodate large parallel misalignment.
Material used: mild steel
Length, L: - 280 mm
L1: - 100 mm
L2: - 80 mm
Diameter of the disks, ØD: 1700 mm
Diameter of the shaft, ØB: 400 mm
Oldham’s coupling is an example of third inversion of double-slider crank mechanism. When link 3, of the double slider crank chain shown in fig. 1.28(c), is fixed and link 1 is free to move, third inversion is obtained which is shown in fig 1.30. In this case each of slide blocks (i.e. link 2 and link 4) can turn about the pins A and B. If one slide block (say link 2) is turned through a definite angle, the frame (i.e. link 1) and the other block (i.e. link 4) must turn through the same angle.
This inversion is used in Oldham’s coupling (shown in fig 1.31) which is used for connecting two parallel shafts when the distance between the two shafts is small. The two shafts to be connected have flanges at their end which are rigidly fastened by forging to the shaft. This flanges from link 2 and 4. Each of these links forms a turning pair with link 3.there is diametrical slot cut in the inner face of these flanges. An intermediate piece is a circular disk (link 1) has two tongues T1 and T2 on each face at right angle to each other. These tongues can slide-fit in the slots in the two flanges (link 2
and 4). The link 1 can slide or reciprocate in the slots in the flanges. Frame and bearings form the link 3 which is fixed.
When the driving shaft is rotated, the flange A (i.e. link 2) connected rigidly to the driving shaft also rotates by the same angle, the intermediate piece also rotate by the same angle through which flange A has rotated. Due to rotation of intermediate shaft, the flange B (i.e. link 4) connected to the driven shaft, also rotate by the same angle. Hence link 2, 4and 1 have the same angular velocity at every instant.
The distance between the axis of the shaft is constant and hence the center of the intermediate piece will describe a circle of diameter equal to the distance between the axes of the shafts. There is a sliding motion between the link 1 and each of the other link 2 and 4.
Maximum shaft speed is:
The maximum sliding speed of each tongue along its slot is equal to the peripheral velocity of the disc along its circular path.
The peripheral velocity of the disk= ω x x
x = distance between the axes of the shaft, and
ω = Angular velocity in rad/s of each shaft.
Oldham Coupling Lubrication Pump
The lubrication pump on GE Reciprocating compressors is usually Directly driven by the compressor Shaft through an Oldham coupling Which transmits the torque from the reciprocating compressor crankshaft to the lube oil pump. In larger and high-speed (>700 RPM) compressors, the power absorbed by the lube oil pump is very high and therefore, the coupling is a very critical item for compressor reliability.
ADVANTAGES OF OLDHAM’S COUPLING
- Protects driven component by serving as a mechanical "fuse" - an inexpensive replaceable plastic midsection shears under excess load
- Protects support bearings by exerting consistently low reactive forces, even under large misalignments
- Homokinetic transmission - driving and driven shafts rotate at exactly the same speed at all times
- Zero backlash and high torsional stiffness
- Electrical insulation
- Accommodates large radial misalignment in a short length
- Easy installation in blind or difficult installations when through-bores are used
- Economically priced compared to other couplings with similar performance characteristics
- Inexpensive replaceable wear element
- Low moment of inertia
- No velocity variation as with universal joints
- High lateral misalignments possible
- High torque capacity
- Ease of dismantling
DISADVANTAGES OF OLDHAM’S
· Limited angular displacement of shafts
· Need for periodic lubrication due to relative sliding motion unless nylon or rubber construction is employed
· Possible loss of loose members during disassembly
- Accommodates a relatively small angular misalignment
We have achieved the goal of studying & analysing the working of the Oldham’s coupling by fabricating a model of the coupling.