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# Coupling Angular Misalignment4

## Coupling Angular Misalignment

(OP)
Hello Experts,

75degree per coupling half

How much this is in terms of mm or inches? The misalignment pertains ti RAZ 75 GEAR COUPLING

### RE: Coupling Angular Misalignment

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

### RE: Coupling Angular Misalignment

Seventy Five Degrees or POINT seven Five?

Either way you can quickly work this out form simple high school trigonometry once you have some dimensions to work from. Which you haven't given us.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Coupling Angular Misalignment

(OP)
Sorry it is 0.75 degree per coupling half. How trigonometry can be used? Please guide

### RE: Coupling Angular Misalignment

You have an angle (0.75 degrees)...Using the dimensions of your coupling and sine/cosine functions you can calculate the discrepancy.

### RE: Coupling Angular Misalignment

4
You get 0.75 degrees per coupling half. But the key thing to understand about misalignment it that all misalignment resolves back to angular misalignment of the coupling flex element. So the angular rating of the coupling flex element in your case is 0.75 degrees, but that is the maximum value of misalignment that that coupling is rated for, NOT the amount of angular misalignment that your shafts are permitted. WHY? This is because all misalignment of your system resolve back to angular misalignment in the coupling flex element. So you have to measure the system parallel misalignment, axial misalignment, and angular misalignment. Those all have to be resolved to angular misalignment in the flex element and ADDED together. THAT summed value needs to be below 0.75 degrees (your coupling rated misalignment)

For example (coupling rated at 0.75 degrees misalignment 5" flex element diameter, 7" Distance between flex planes)

Alignment values
0.01" angular misalignment
0.04" parallel misalignment (root squared sum in horizontal & vertical)
0.02" axial misalignment

Angluar calc
[InvSIN(0.01/2.5)]/2= 0.11 degrees

Parrallel calc
InvSIN(0.04/7) = 0.32 degrees

Axial calc
[InvSIN(0.02/2.5)]/2 = 0.23 degrees

Total misalignment in application = 0.11 + 0.32 +0.23 = 0.66 degrees
This is close to the rated misalignment of the coupling, a little too close if you ask me. WHY? because now we need to consider dynamic misalignment. What happens when the application sees torque and thermal growth. This can cause further misalignment. This can be very hard to measure but must be considered.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Coupling Angular Misalignment

There's a man who knows his couplings....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Coupling Angular Misalignment

For op, I'd bring up one additional point: the coupling manufacturer's alignment tolerance (even when properly interpretted per CouplingGuru's instruction) is not necessarily what you want to use as an alignment tolerance for your machine. All the coupling manufacture's tolerance tells you is what the coupling is designed to withstand long-term, it says nothing about reaction forces and their long-term effect on sensitive machine components (bearings, possibly bolted joints in the load path). Many users select a target alignment tolerance based on machine speed (similar to this approach https://vibralign.com/other-topics/know-your-align... ), and adjust it downward only if the coupling manufacturer's tolerance is lower. (I'd be interested to hear CouplingGuru comments on what I said if anything sounds wrong)

CouplingGuru - can you explain how shaft axial misalignment translates to angular misalignment of a coupling half?

Also, in your example, the misalignment values listed would be square root of sum of squares of those for the two radial directions, right?(for example 0.04" parallel misalignment listed could have resulted from parallel misalignments of 0.025" and 0.030" in the horizontal and vertical directions)

=====================================
(2B)+(2B)' ?

### RE: Coupling Angular Misalignment

Electricpete,

You are exactly correct, but all couplings are not created equal when it comes to restoring forces (reactionary forces). A couplings ability to withstand misalignment is mainly predicated on the design and material selection of the flex element. so although the fatigue limit of the flex element will not be reached, that doesn't mean the adverse reactionary forces generated are acceptable. Gear couplings are particularly challenging because the more torque you drive through them, the less willing they are to misalign. This is due to the fact that torque is driven through the misalignment plane via friction. higher torque driven = high force required to overcome friction. Disc couplings are different, torque is driven through 1 plane, misalignment occurs through another. So no matter what torque you drive though a disc coupling the reactionary force are virtually unchanged. But the stress strain relationship in a disc coupling flex element changes as high torques are driven, so some disc coupling manufactures advertise high misalignment, but that is at high service factors (low torque) as you lower the service factor (drive more torque) the misalignment rating has to get de-rated. So no matter what any disc coupling manufacturer claims, all disc couplings at a given diameter have about the same amount of misalignment capability. The only way to increase misalignment in a disc coupling is to have more discs to lower the stress in the disc pack. Then you are working that stress strain relationship, trading stress for strain. However, that comes at a price, higher reactionary forces. More discs is a thicker flex element pack which leads to higher reactionary forces. So it is a balancing act.

Second point, axial misalignment to angular misalignment.
This is a little tricky, a coupling by design can only bend out of alignment so far until it hits a dead stop. So axial misalignment changes where that dead stop will occur. that can be a concern it is basically limiting the amount of movement that is available, while this isn't as critical in a gear coupling, in a disc coupling it can be very critical.
However, the more concerning aspect of of axial misalignment applies to 1 part flex elements (flex elements that function off a stress strain relationship) like discs or elastomers. Axial misalignment in these 1 part flex elements kind of works like a pre-load on a beam. For example, assume loading a beam cyclically with 1000 lb force generates a fatigue limit over time. Then, loading that same beam cyclically with a 1000 lb force AFTER a 250 lb constant force is applied, will generate a shorter fatigue limit then if the 250lb static force wasn't present. That 250 lb force is acting like axial misalignment. axial misalignment in 1 part flex elements is the pre-strain present in the system. The easiest safest way to account for it is resolve it back to angular misalignment per flex element.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Coupling Angular Misalignment

"There's a man who knows his couplings...."

User name checks out.

### RE: Coupling Angular Misalignment

A little off the discussion.

If your experiencing maintenance issues or coupling failures a gear coupling may not be your primary fix. I have seen dozens of coupling failures over the years and usually it's an alignment and or lubrication issue.

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