Are You Using Coupling Tolerances to Align Your Machines? You’re Doing It Wrong
By John Lambert
If we want to get into precision maintenance, we need to be aware of the differences between shaft alignment tolerances and coupling tolerances.
June 22, 2020
By John Lambert
Precision maintenance isn’t new. Apparently, the term precision maintenance was first used by actual rocket scientists who worked for NASA in the 1960s. I first heard the term from Ralph Buscarello of Update International; he taught me shaft alignment using dial indicators almost 40 years ago.
What is precision maintenance? If you look up precision in the dictionary, it says:
1. The state or quality of being precise; exactness.
2. The ability of a measurement to be consistently reproduced.
The two key words are exactness and reproduced. As we know, a carpenter measures twice before cutting once. If the measurements are not the same (reproduced), he does not cut until he answers the question Why?
Exactness makes statements such as “that’s close enough” redundant because we do not want “close”; we want it to be within tolerance.
That is how I define precision maintenance. It’s simply working to a tolerance. The reason we must work to an accepted tolerance range and not to an “exactness” is because in an environment of rotating machinery, machines and mechanical parts are constantly subjugated to constant forces in which an exact measurement can’t be fulfilled. The problem is, some people get confused by some of the tolerances that are out in the maintenance world, especially when it comes to shaft alignment and major mechanical parts involved.
In rotating machines, the power transfer from the driver machine to the driven machine is done through a coupling.
These are amazing mechanical components that must endure high stress levels from horsepower, torque, and process-load demands.
Most of the couplings used in a motor-to-pump setup are flexible couplings. As the name implies, they flex, allowing for small amounts of misalignment. The amount of misalignment they can tolerate depends on the design of the coupling.
There are a lot of different styles of flexible couplings, including gear, jaw, grid, chain, and disc. Each has its benefits, and some work better in different applications. For smaller pumps, we normally see elastomeric couplings; for example, a jaw coupling, shown in Figure 1. These are popular because they can be quickly opened to do an alignment or replace the elastomer.
For larger pumping systems we see gear couplings (Figure 2), but regardless of the style, they are all flexible (to a point) couplings. Couplings are one of the biggest workhorses in the power transmission industry. When we measure shaft misalignment, we must consider both types of misalignment: the offset (also known as parallel offset) between the two centrelines of the shafts and the angle between the two-shaft centrelines (illustrated in Figure 3).
This means, when you see a tolerance chart or table, you will see offset and angular values. It is the same with couplings; they will give you a tolerance for both offset and an angle.
Coupling Tolerances vs. ANSI Tolerances
The Lovejoy S-Flex coupling is a short flex coupling, and the offset and angle are given in their misalignment capability guide for each coupling: Parallel, the S-Flex Endurance coupling accepts up to .062 inches of parallel misalignment. Angular, the S-Flex Endurance coupling accepts angular misalignment up to one degree.
In this instance, the angle has been given as a degree. We can easily convert that to thousandths of an inch (thou), per inch. One degree is 0.01745” (thou) per inch. That means, on a two-inch coupling, you would be allowed a gap at the top or bottom and side to side of 0.035” thou.
Figure 4 shows a shaft alignment tolerance chart from the American National Standard Institute (ANSI) Shaft Alignment Methodology, Part 1: General Principles, Methods, Practices, and Tolerances. The statement above the table shows that the RPM is the deciding factor in shaft-to-shaft tolerances. The faster the machine runs, the tighter the tolerance.
If you look at the tolerances, you will see it written in metric and imperial. In the brackets, the tolerance is in mils. A mil is the same as a thou, it’s just written differently. One mil equals one thou — i.e., 1.0 mil = 0.001 inch (thou). Mils are used throughout North America, although mainly in the U.S., as this is an ANSI standard.
If we look at the table under 1,800 RPM, which is the most common RPM in the industry, it shows the allowable tolerances divided into three ranges:
Minimal – 6.0 mils of offset and 1.2 mils per inch of angle.
Standard – 3.0 mils of offset and 0.6 mils per inch of angle.
Precision – 1.6 mils of offset and 0.3 mils per inch of angle.
When comparing the minimal ANSI tolerances to the tolerances of the jaw coupling in thou:
– The coupling tolerance is 0.062” of offset, 0.01745” per inch of angle.
– The ANSI standard is 0.006” of offset, 0.0012” per inch of angle.
Why the big difference? The coupling manufacturer is not trying to mislead you; they are simply pointing out that their coupling can withstand a high degree of misalignment, which is a great benefit to the user. The fact that the bearings in the machines cannot handle the high vibration created from too much misalignment is not their concern. It’s yours.
And the reason it’s a benefit to you is that many machines have to be offset, or misaligned, when installed so they can expand into alignment when their at operating temperature (thermal growth). This means, a coupling that can handle large amounts of misalignment is needed. In other words, the specification sheet you get with your coupling is for the couplings, not for shaft-to-shaft alignment tolerances.
While most coupling manufacturers give good information, some seem to be on the outrageous side. I have seen one that claimed an offset of one-eighth of an inch and four degrees of angle. Even if it has a rubber tire for an insert, it will have very high vibration levels that would destroy the bearing, seals, and eventually the machines it is attached to.
During our alignment training we spend a lot of time on couplings because they have a large influence on alignment work. One of the most common questions asked is “Should the couplings be loose or open when we align them?”
Couplings that are installed (coupled together) when the machine is misaligned can lock up and not flex when the shafts are rotated. This can give incorrect results when aligning with a dial indicator or laser system.
We actually promote open-coupling (disengaged) alignment so that we remove any potential strain from the coupling. Some couplings are quite stiff and will deflect the shafts.
Figure 5 shows a blower that is being aligned. The worker who’s doing the job is only using half the coupling elastomeric link to reduce the shaft deflection. It’s a good coupling and this was their solution to avoiding potential coupling interference.
The bottom line is, with precision maintenance, you have to make sure you get the right tolerances for your machines and follow the correct alignment procedures. MRO
John Lambert is the President of BENCHMARK PDM. He can be reached at firstname.lastname@example.org.