My question is for correctly finding a inch measurement when given a degree as a starting point. Example, a coupling spec states that the amount of angular alignment allowed is 3 degrees. The skf laser alignment tool displays in inch/mm. How does one convert the 3 degrees into a inch/mm measurement.

Thanks

Hmmm...maybe start with concepts learned in high school trigonometry?

John from PA

Xracer
Not familiar with SKF laser tool, but I know that my laser kit has tolerances based on delta between top and bottom for vertical angularity (or side to side for horizontal) of the coupling diameter. I suspect the reason for this is that this was the easiest way for a mechanic to measure angularity since there was no "angularity" tool. The coupling manufacturer simply provided a maximum "delta" at the edge of the coupling for two measurements 180 deg apart. Sometimes this measurement is called gap. So two shafts perfectly aligned (at least for angularity) would have zero delta. Because of the right hand rule, angularity of the coupling is same as angularity between shaft centerlines

Since not all couplings have the same diameter, tolerances are sometimes normalized to a specified diameter of coupling. My alignment tool has tolerances of mils/10 inches. Not sure why SKF chooses to mix English and metric units. In any case, whenever discussing angularity tolerances, make sure you understand this distinction since coupling manufacturers dont usually normalize the tolerance - they simply state the requirement for that particular sized coupling.

If you understanding the discussion above, then the angularity in degrees is simple geometry.
If I remember my geometry correctly, the sine of 3 degrees will approximate the tolerance (delta gap(mm)/mm). Now you simply have to divide the delta gap by 25.4 mm/inch to make units come out to be inches/mm

By the way, 3 degrees angularity seems high to me. You could align to meet coupling manufacturers criteria and still put large loads on bearings.

Jim P

3 degrees sounds like a tolerance published by a flexible coupling manufacturer. These specs are written with only the coupling in mind, not the bearing and very often are referred to as "static misalignment" tolerances.

Use the speed based specs in the alignment system, not the coupling specs.

John,

Where is your sensitivity? This looks like it is from a fellow Yankee.

I'd prefer dealing with inch/inch or mils/inch instead of in/mm.

If you have a tolerance of 1 minute for example, you take the tan of the angle times distance spanned. So, if you are spanning 10 inches X 0.00029 or ~0.0003 - then you are allowed 3 mils.

So 3 degrees is 180 times 1 minute. Basically anyone can eyeball 3 degrees (0.0524 = tan). So 1/2" / 10" basically is OK!! I think NOT.

Three degrees is high in anyone's book.

3 deg. is almost good for plumbing. 3/4"/1 ft.

For small angles, which is what is generally used for alignment, multiply the angle in degrees by 0.0009139, and you will be really close. Be sure to use degrees in decimal form if needed. This gives inch/inch, or is that m/m?

Thank you all for your replies. As a standard, I have been using the alignment guide that is provided with the alignment tool. I do like to compare the values with that of the coupling being used. Some coupling manufactures have some higher tolerances, while others use the "gross alignment" etc.

Oh, John from PA, thanks for the reply.. Some have a higher retentivity than others. Yes I did have trig in high school, but again, that was a long time ago and the saying goes, use it or lose it.

Your not saying that John is a#$l retentive are you?

tan(theta)=misalignment/coupling dia
=shim thickness/distance between motor feet.

where theta=3 degrees.
If you know coulping dia in inch/mm , you can get
misalignment in inch/mm. Also, you can calculate shim thickness required for correction of misalignment by above formula.
Regards.
By misalignment, I mean angular misalignment.
Of course, ti is valid only if there is no angular float.

This message has been edited. Last edited by: AKHTAR,

A word of caution -- the coupling diameter may not have anything to do with the angular displacement of actual readings taken. Few use the coupling and if there is run-out in the coupling a physical measurement by mics or calibers will have error.

The distance swept by an indicator or the span is a better approach. However, if you are using the Rim & Face method, the face reading is an actual value: but, if you are using Indicator Reverse the delta between the indicators /2 will give actual value. Always ensure there is no axial float or you are compensating for it. Dynamic readings ensures run-out is not a factor.

The above formula posted is incorrect; or it doesn't translate into English and isn't well defined to the extent that it is valid.

This post is not to admonish anyone but rather to prevent someone from an incorrect setup.

quote:

Originally posted by William_C._Foiles:
Your not saying that John is a#$l retentive are you?

No, I'm not saying that. I'm just saying that its nice that he can remember high school trig but others many not recall what was learned years ago.. Like it has been said, if you don't use it, you will lose it.

quote:

Originally posted by Danny Harvey:
3 degrees sounds like a tolerance published by a flexible coupling manufacturer. These specs are written with only the coupling in mind, not the bearing and very often are referred to as "static misalignment" tolerances.

Use the speed based specs in the alignment system, not the coupling specs.

Yes, the 3 deg are from the coupling specs. Thanks for clearing up the differences.

Xracer, my thought was if I send you down a path then you will learn by discovering the answer to your problem.

What everyone here has contributed is great, but someone else basically solved your problem in that you learned that 3 deg is probably too high. But the next time, when the question arises about 1.8 degrees for example, will you be able to work out a solution, or will you again come to this board for an answer? I'd like to think that we contribute "help" and not "answers." You develop your own answers. Remember, you are there with your machines, and we are miles away, very often with very limited information.

John from PA

The correct answer is that the alignment tolerance should NEVER be the coupling limits. Offsets at the power transmission points (mils/inch of coupling span distance) are a good place to start.

If all you have is "forgotten" high school Trig, you might consider hiring an Engineer. John would be a good one if he is not too busy and you could afford him.....

By the way, the real secret to success for alignment and balance is an anal retentive mindset! He who makes the best measurements and keeps the best notes always wins.

John was after some type of zen stuff. Feed a man and he will be full for a while. Teach him to farm, and his hands will be dirty for life.