Power Transmission by Flexible Couplings

The flexible couplings allow certain amount of parallel and angular misalignments and absorbs shock forces and hence they are preferred for many direct drives. The resilient tyre flexible coupling finds wide acceptance due to their desirable features of easy installation and maintenance. The details required for selection of coupling are:

1. Type of driven machine and operation hours per day.
2. Speed and power absorbed by the drive machine (power rating of the drive motor).
3. Diameter of shaft to be connected.

The following procedure is followed for selection of flexible coupling:

1. Service Factor:  Determine the required service factor from the following table:

   	Type of driving unit
   	Electric motor steam turbines			Internal combustion engines, steam engine water turbines
   Operation hours per day	Less than 10	Over 10 upto 16	More than 16	Less than 10	Over 10 upto 16	More than 16
   Class 1: Agitators, centrifugal compressors and pumps / fans upto 7.5 kW, blowers and generators	0.8	0.9	1.0	1.3	1.4	1.5
   Class 2: General machine tools, rotary pumps, fans over 7.5 KW	1.3	1.4	1.5	1.8	1.9	2.0
   Class 3: Bucket elevators, cooling tower fans, piston compressors and pumps	1.8	1.9	2.0	2.3	2.4	2.5
   Class 4: Reciprocating conveyors, vibratory screens	2.3	2.4	2.5	2.8	2.9	3.0

2. Design Power:  multiply the normal running power by the service factor. This gives the Design power which is used as a basis for selection of the coupling.

3. Coupling size:  Refer to the following table and from appropriate speed read across until a power greater than that required in step (2) is found. The size of the flexible coupling required is given at the head of that column. The coupling bore size availability is also to be checked.

   	Coupling Size
   Speed (rpm)	F40	F45	F50	F60	F70	F80	F85	F90	F100	F110	F120	F140
   720	1.58	2.80	4.00	7.99	12.20	19.10	23.00	27.50	38.10	53.70	89.10	142.00
   960	2.11	3.74	5.33	10.70	16.30	25.40	30.70	36.70	50.80	71.60	119.00	189.00
   1440	3.17	5.61	7.99	16.00	24.40	38.10	46.00	55.00	76.10	107.00	178.00	284.00
   2880	6.33	11.23	16.00	32.00	48.90	76.30	92.10	110.00

   Note

   The F40 coupling can take up a parallel misalignment of 1.1 mm whereas F140 can take upto 3.7 mm. End float varies from 1.3 mm to 4.6 mm. The F40 coupling can be used upto 4500 rpm and F140 upto 1800 rpm.

Torque Limiters (Couplings): An improved version of flexible coupling is the torque limiters. It gives overload protection to the equipments. The coupling is present for a given torque requirement of the driven equipment such as refrigeration compressors. Where the torque requirement of the driven machine increases due to some problems in the machine, the torque limitor gets disengaged and driver (such as motor) from the driven equipment and automatically engages back when overload is relieved.