Re: Coupling Failure

Special Instruction 
Maintenance and Inspection of the Rubber Element for the 243-1520 Flexible Coupling Gp, the 247-0308 Flexible Coupling Gp, the 296-8050 Flexible Coupling Gp, the 309-1565 Flexible Coupling Gp, the 370-3967 Flexible Coupling Gp, and the 370-3968 Flexible Coupling Gp. {1000, 3252, 4450}

Media Number -REHS3401-02Publication Date -14/08/2013Date Updated -10/03/2017


Maintenance and Inspection of the Rubber Element for the 243-1520 Flexible Coupling Gp, the 247-0308Flexible Coupling Gp, the 296-8050 Flexible Coupling Gp, the 309-1565 Flexible Coupling Gp, the 370-3967Flexible Coupling Gp, and the 370-3968 Flexible Coupling Gp. {1000, 3252, 4450}

SMCS - 1000; 3252; 4450

Engine 3508 (S/N: CNF1-UP; CTF1-UP; PTK1-UP; 5XM1-UP; 3LS1-UP; CAW1-UP; 5PW1-UP; 95Y1-UP; 1JZ1-UP; 23Z1-UP; 68Z1-UP; 70Z1-UP) 3508B (S/N: S2A1-UP; CNB1-UP; S2B1-UP; CTC1-UP; PTJ1-UP; 2BM1-UP; 3DM1-UP; 4GM1-UP; 7SM1-UP; PTN1-UP; 6PN1-UP; 1TW1-UP; 2HW1-UP; 3DW1-UP; 5KW1-UP; BGX1-UP; 1FZ1-UP) 3512 (S/N: CTE1-UP; PTG1-UP; CMJ1-UP; FDR1-UP; 3MS1-UP; 6WW1-UP; 49Y1-UP; 50Y1-UP; 1KZ1-UP; 24Z1-UP; 65Z1-UP; 67Z1-UP) 3512B (S/N: CTB1-UP; CMC1-UP; FDE1-UP; BLF1-UP; PTF1-UP; S2G1-UP; CTH1-UP; S2H1-UP; BRK1-UP; PTM1-UP; 7HM1-UP; 8EM1-UP; 8RM1-UP; 4TN1-UP; 6WN1-UP; 1PW1-UP; 2GW1-UP; 4AW1-UP; 5AW1-UP; 1GZ1-UP; 2EZ1-UP) 3512C (S/N: LLA1-UP; LLD1-UP; RMS1-UP) 3516 (S/N: CMD1-UP; CTD1-UP; PTD1-UP; 4XF1-UP; 5SJ1-UP; CAL1-UP; 7KM1-UP; GZR1-UP; 3NS1-UP; RKT1-UP; 8TW1-UP; GZX1-UP; 1LZ1-UP; 25Z1-UP; 27Z1-UP; 71Z1-UP; 73Z1-UP) 3516B (S/N: CPA1-UP; CTA1-UP; PTA1-UP; BMB1-UP; PTC1-UP; CDE1-UP; CTG1-UP; BPJ1-UP; CTJ1-UP; LEK1-UP; CBN1-UP; FDN1-UP; 6HN1-UP; 7RN1-UP; 8CN1-UP; 8KN1-UP; 9AN1-UP; S2P1-UP; GAR1-UP; PBR1-UP; S2R1-UP; GZS1-UP; GZT1-UP; 1NW1-UP; 2FW1-UP; 2JW1-UP; 3CW1-UP; 4BW1-UP; 8NW1-UP; FDX1-UP; GZY1-UP; S2Z1-UP; 1HZ1-UP) G3508 (S/N: 4WD1-UP; 2JF1-UP; 9TG1-UP; CPJ1-UP; CTN1-UP; CPY1-UP) G3512 (S/N: 5JD1-UP; 7NJ1-UP; CTM1-UP; CRN1-UP; CSP1-UP; 5ZZ1-UP) G3512E (S/N: GHD1-UP; GHF1-UP; SSR1-UP; SST1-UP) G3516 (S/N: ZBA1-UP; 8LD1-UP; CPG1-UP; 4EK1-UP; CTL1-UP; CSZ1-UP; ZAZ1-UP) G3516B (S/N: ZBB1-UP; ZBC1-UP; CME1-UP; CTW1-UP; 7EZ1-UP) G3516C (S/N: RWA1-UP; GSB1-UP; TJB1-UP; TJC1-UP; DKR1-UP; GZZ1-UP) G3516E (S/N: GHP1-UP; GAS1-UP; SLY1-UP) G3516H (S/N: NWJ1-UP; GLN1-UP) G3520B (S/N: CWD1-UP; CTK1-UP; GZP1-UP) G3520C (S/N: GDB1-UP; GHC1-UP; MAD1-UP; GHE1-UP; GZJ1-UP; GZK1-UP; GZL1-UP; GZM1-UP; GZN1-UP; B9P1-UP; LGS1-UP; HAT1-UP; CWW1-UP; JBX1-UP; CWY1-UP; JBZ1-UP) G3520E (S/N: GZG1-UP; GZH1-UP; HAL1-UP; GHM1-UP; GHR1-UP; SXY1-UP) G3520H (S/N: GFP1-UP; GFR1-UP) G3524 (S/N: CRC1-UP) G3532 (S/N: CPF1-UP; 8BZ1-UP) Petroleum Engine 3508C PETROLEUM (S/N: LLC1-UP) 3512C PETROLEUM (S/N: LLB1-UP) 3516C PETROLEUM (S/N: PES1-UP) Power Module PM3508 (S/N: BLM1-UP; BPY1-UP) PM3516 (S/N: BPD1-UP)



Do not perform any procedure in this Special Instruction until you read this information and you understand this information.

This Special Instruction is to provide information about the maintenance and the inspection of the rubber element for the 243-1520 Flexible Coupling Gp, the 247-0308Flexible Coupling Gp, the 296-8050 Flexible Coupling Gp, the 309-1565 Flexible Coupling Gp, the 370-3967 Flexible Coupling Gp, and the 370-3968 Flexible Coupling Gp.

Most torsional couplings use one or more torsional elements that are formed from rubber or from other polymers with similar physical characteristics. The torsional couplings are placed in series to achieve exact stiffness for reducing torsional vibration. The torsional couplings are also used for adding tolerance to parallel offset. These types of rubber couplings are used in generator sets.


Required Tools


Table 1

QTY Part Number Description 
448-0722 Probe 
448-0723 Probe 
448-0724 Tool Gp 
448-0725 Tool Gp 



General Care

The following factors will accelerate deterioration of rubber:



  • Age


  • Repetitive Starting and Stopping of Generator Set


  • Harsh Electrical Events (such as loss of synchronism) when paralleled with utility


  • Heat


  • Ultraviolet light (UV light)


  • Petroleum products


  • Solvents


  • Anaerobic adhesives (thread lock compound)


  • Ozone

Anaerobic adhesives can also deteriorate the bond between rubber elements and the steel flange or the hub.

In an environment that is free of an aggressive chemical and light related agents, the main cause of an accelerated deterioration of the rubber is heat.

There are five causes of heat buildup in rubber coupling elements:



  • High ambient temperature


  • Damping of torsional vibrations


  • Compensation for angular misalignment


  • Compensation for radial misalignment or for offset


  • Compensation for axial misalignment or for movement

In a controlled environment, the destructive conditions can be minimized. The rubber couplings can have a long useful life.

View Image

Illustration 1g01322929
Flexible coupling
(1) Hardness stamp 
(2) Outer coupling 
(3) Rubber element



Preventive Maintenance Schedule

Maintenance for the Rubber Flexible Coupling consists of four activities:



  • Keep the coupling free of excessive heat, UV light, petroleum products, solvents, anaerobic adhesives, and ozone.


  • Periodically inspect the rubber and the metal components for any imperfections, for stress, and for wear.


  • Check the torque for the fasteners periodically.


  • Check the alignment periodically.

View Image

Illustration 2g01323258
(1) Rubber areas that are peeling away from the steel hub 
(2) Wear at the loaded side of the teeth

View Image

Illustration 3g01323319
Detail A
Cracks in the rubber

During visual inspection of the rubber element, note any rubber areas that are peeling away from the steel hub. A wear of 3 to 4 mm (0.118 to 0.157 inch) at the loaded side of the teeth is allowable. Refer to Illustration 2.

During visual inspection of the rubber element, note any tears or cracks in the center of the rubber element. Refer to Illustration 3.


Inspection Schedule


Every 2,000 Hours

Inspect all externally visible surfaces and check the torque of accessible bolts. Refer to the "Inspection" section.


At Top End Overhaul (8000 to 22000 Hours)

Unbolt the drive ring from the flywheel. Slide the drive ring away from the flywheel over the rubber element. Inspect the surfaces of the drive of the rubber element and the back side of the rubber element. A borescope can be used to check for cracks on the flywheel side of the rubber element.




Torsional Set

View Image

Illustration 4g01323355
Torsional set of rubber coupling
(X) Distance of torsional set 
(Y) Diameter of molded edge

Use the following steps to determine the amount of torsional set of a rubber coupling. Refer to Illustration 4.


  1. Use a straight edge and the centerlines of the holes on the inner hub. Put a mark on the edge of the mold line of the rubber element.


  2. Use a straight edge and the centerline of a tooth space. Put a mark on the edge of the mold line that is on the rubber element.


  3. Measure the distance between the marks. Replace an element when the torsional set (X) exceeds 25 mm (1 inch).



Examine all rubber surfaces for cracks. Refer to Illustration 3.

Length - Acceptable hairline cracks are random and short. Cracks that radiate diagonally from the center of the rubber element indicate that the rubber element has failed. The rubber element must be replaced.

Depth - The rubber element must be replaced if there are cracks that exceed 3 mm (0.12 inch).

Width - Walls that are clearly visible from the bottom of crack indicate that the rubber is seriously hardened or the rubber is damaged. Replace the rubber element.


Tooth Wear

3 to 4 mm (0.12 to 0.16 inch) of wear on the loaded edge of the tooth on the rubber element is allowable. It is also normal for there to be a gap of a few millimeters between the rubber and the drive ring. Centrifugal force stretches the rubber element to contact the drive ring in normal operation.


Bond Lines

A bond line is the joint between the metal hub and the rubber element. Examine this area for separation. If separation that is not clearly an air space from the manufacturing process is present, the rubber element must be replaced.


Bulges or Swelling

Any bulge or any swell that is larger than 6 cm2 (2.4 inch2) requires replacement of the rubber element.


Rubber Hardness

Use a durometer. Use the Shore A hardness scale. Check the rubber hardness in several locations on the element. The rubber hardness should be within ± 3 of the shore hardness rating of the element

Super Contributor

Re: Coupling Failure

Reverse power problems would have to be severe to cause issues with these couplings.


Most of these type failures I have seen are due to alignment issues, more prevelant with medium voltage tail ends. My personal opinion is that the frame rails for the larger generator ends do not properly support the package, reducing coupling element life. I also think the CAT procedure for alignment of the close coupled two bearing generator does not provide the proper alignment in all cases, have done the procedure per CAT method and rechecked with a laser tool and found significant misalignment in many cases. Well performed vibration analysis can help determine if this issue is present.


Coupling elements should be regularly inspected, in most cases they aren't until a problem occurs. In most cases a visual inspection thru the fan guard is adequate.  You should also be monitoring the front and rear bearing temperatures as well, the difference between the front and rear (DE and NDE) bearing should be similar to the difference in air temperature between the cooling air into the rear of the generator and the exhausted air out at the fan/coupling guard.


Here is a link to a Centamax S installation and maintenance manual,




Re: Coupling Failure

does reverse power has any effect on the rubber coupling??

Trusted Contributor

Re: Coupling Failure

Excessive heat.

mis alignment.

out of spec torsional vibrations.

synchronisation issues.

all can contribute to coupling failure.



Re: Coupling Failure

i had change the coupling with a used one , but till now i didn't find clear reason for the damage


Re: Coupling Failure

do you have any news about your failure because i have exactly the same problem 


Re: Coupling Failure

do you have news for you failure coupling ?



Re: Coupling Failure


Super Contributor

Re: Coupling Failure

This appears to be a Centamax-S coupling, I would suggest you contact Centa for technical assistance, this is the contact I use,


Be preparde to provide details for your appilcation such as power rating, nominal engine speed, ambient conditions, load factor and alignment information.




Re: Coupling Failure