May 26, 2015
Where is the Center of the Coupling?
By James Pekarek
During a precision laser shaft alignment, we measure the setup dimensions and input these into the Fixturlaser Laser System Display Unit. The requisite dimensions are Stationary Sensor to Movable Sensor, coupling center to M Sensor, M Sensor to movable inboard foot, and foot to foot on the moveable machine.
One dimension that shaft aligners occasionally have questions about is the “Coupling Center to Moveable Sensor”. On certain types of couplings, it may be difficult to determine the correct coupling center to determine that dimension.
On a recent training trip to the Sacramento area, we worked with a machine with a very large, compression style coupling. It was difficult to determine the coupling center and it brought up several questions from the students. The students had differing ideas about where the “Coupling Center” was. The three questions that stand out are 1) Is the center between the two sensors? 2) Is the center between the two shafts? 3) Is the center between the coupling halves?
Surprisingly, the answer may be all of these, or it may not necessarily be any of these. The most correct answer would be “between the coupling halves”, however it may be difficult to determine where that actually is when working with some types of couplings.
This training class was performing a precision shaft alignment on 2MW engine driven genset with a 30” diameter, Renold DCB flexible coupling (pictures 1 and 2). This style of failsafe coupling has overlapping inner and outer elements with compression blocks transferring the power from the outer element (mounted on the engine flywheel) to the inner element (mounted to the generator shaft).
Photos 1 & 2
As the coupling “hubs” overlap each other (photos 3 & 4) and the sensor setup is non-symmetrical (S sensor on engine flywheel with M sensor on generator shaft) the class was unsure of what dimension to us for the “coupling center” to M Sensor.
Photos 3 & 4
The “center” of the coupling typically is defined as the center point of the flex element between the two coupling hubs. Photo 5 shows the mounting position of the sensors, the center point between the sensors is not the same as the center of the coupling element since the sensors are mounted offset in relation to the coupling hubs.
So for this set up the center of the coupling was determined to be the center of the outer coupling element as indicated by the “pivot point” on photo 3.
If the distance from the “center of the coupling” to the M Sensor cannot be easily determined and you are slightly off with this dimension do not stress out, you will still get excellent accurate alignment results!