Shaft alignment spec on small machine
Shaft alignment spec on small machine
(OP)
I am involved in the design of a device that utilizes a rotating shaft driven by a T-head gearbox to two driven devices on each end of the shaft. All three of these devices are mounted to a common plate. Note that the shaft rotates slowly. Approx 100 rpm. The shaft diameter is approx 1/2"... plate ground flat... This is a small benchtop machine and the devices weigh less then 20 lbs.
The discussion point is regarding the "need" for specifying a method to align the two driven devices to the T-head gearbox (and shaft). One recommendation is to "simply" torque all mounting fasteners as a group once fully assembled. Effectively using the shaft to align the driven devices.
Ultimately the debate is:
Does there need to be any specifics on the face of the drawing to ensure the shaft alignment process takes place as described above? Or are there industry standard / millwright standards etc that can be assumed will be followed.
I realize I am posting this in a drafting forum amongst engineers. I expect an overwhelming "we need to define everything," and "never assume." However I am playing a little bit of devil's advocate and curious if the task doing this sort of assembly should carry the expectation that qualified assemblers would just know to ensure alignment via whatever background training (qualification) they have. Similar to tightening lug nuts or head bolts.
I have spent the better part of the day trying to find examples (online) of machinery assembly drawings to make a case either way. However as would be expected, companies don't have their proprietary documentation readily available.
Looking for feedback from other industry professionals. Thanks.
The discussion point is regarding the "need" for specifying a method to align the two driven devices to the T-head gearbox (and shaft). One recommendation is to "simply" torque all mounting fasteners as a group once fully assembled. Effectively using the shaft to align the driven devices.
Ultimately the debate is:
Does there need to be any specifics on the face of the drawing to ensure the shaft alignment process takes place as described above? Or are there industry standard / millwright standards etc that can be assumed will be followed.
I realize I am posting this in a drafting forum amongst engineers. I expect an overwhelming "we need to define everything," and "never assume." However I am playing a little bit of devil's advocate and curious if the task doing this sort of assembly should carry the expectation that qualified assemblers would just know to ensure alignment via whatever background training (qualification) they have. Similar to tightening lug nuts or head bolts.
I have spent the better part of the day trying to find examples (online) of machinery assembly drawings to make a case either way. However as would be expected, companies don't have their proprietary documentation readily available.
Looking for feedback from other industry professionals. Thanks.
-Dustin
Professional Engineer
Pretty good with SolidWorks





RE: Shaft alignment spec on small machine
This is more of a mechanical engineering problem than a dimensioning and tolerancing one. How much mis-alignment can you tolerate? How capable are your assembly people?
You can use your shafts for alignment, but I would expect things to shift when you tighten stuff down.
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JHG
RE: Shaft alignment spec on small machine
Ultimately, I am looking for either examples of drawings that specify a process for something like "tightening head bolts" or examples where that type of process definition is omitted because the installers are expected to already know the process.
If this assembly process were "more complicated" or more frequently used by varying devices, we would likely create a standalone internal document to define the process. The drawing would typically contain a note stating "Align IAW 1020863." But where do you draw the line on defining a process for a task that is viewed by many as an expected understanding.
I realize the conservative approach is to include a note of some sort. But is it bad that a note was not present? Are there examples of a similar process where a note was not present? Is there overwhelming evidence for the process being defined?
-Dustin
Professional Engineer
Pretty good with SolidWorks
RE: Shaft alignment spec on small machine
If you can assemble the thing and get it working, there is a way to document the process. If the
instruction is simple, it can be a note on the drawing. At some point, you need the power of a word processor to write stuff down. Specifying a minimum level of skill by the worker is done all the time in manufacturing. Skilled workers are of course, more expensive.
I am just picking numbers out of the air here. Take the case that you cannot align your shafts to any better than .001". When your machine runs, your shafts will be strained ±.001" as they rotate. What is the resulting stress, and what is your fatigue life going to be?
Can you make your shafts longer?
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JHG
RE: Shaft alignment spec on small machine
I have found some larger machinery recommendations (facility pump install and alignment type stuff). However those are typically installed by millwrights who do have a high level of training is the area of alignments. At that level (I could be wrong), I believe the process are not defined. Perhaps just the goal. I say perhaps, because the bit of research I have done leaves that answer vague.
I am not finding examples of what other companies or industries do for their assemblies where a process slightly more complicated than a torquing lug nuts (single part with several fixed fasteners).
Input certainly appreciated.
-Dustin
Professional Engineer
Pretty good with SolidWorks
RE: Shaft alignment spec on small machine
Documenting an/the assembly procedure is, to me, a basic DFMA practice. If you have a plan to get the thing assembled and working, then we can expect the thing to be assembled and got working. If manufacturing can find a better way, that is nice. If manufacturing can find no way...
If I were checking your drawings, I would want to know how your shafts got aligned. Either you show me flexible couplings, accurate enough and manufacturable tolerances, or an assembly procedure. Another possibility is that your company routinely does this, and your assembly people are accustomed to dealing with it.
Do these shafts ever break?
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JHG
RE: Shaft alignment spec on small machine
In hindsite, having a note (process) is the right answer... but how wrong was it that the process wasn't defined.
I am an intermediary in this activity. Not the designer, not the assembler.
-Dustin
Professional Engineer
Pretty good with SolidWorks
RE: Shaft alignment spec on small machine
Is this production, or a one-time-only assembly. In production, IMHO, full documentation is part of due diligence. If this is limited production or one-off tooling, assembling the first one and showing everyone how to do it should be sufficient. We are moving away from engineering here, and into the realm of office politics. How well do the designer and the manufacturing engineer get along?
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JHG
RE: Shaft alignment spec on small machine
Being in the middle, is the reason I was hoping for concrete examples one way or the other. Still hoping if anyone can provide. Not looking for actual drawings, just further opinions or examples of how their industry works.
-Dustin
Professional Engineer
Pretty good with SolidWorks
RE: Shaft alignment spec on small machine
When you quoted me, you left out "IMHO". It's important.
I was recently laid off from a place that generally does not give a s**t about stuff like that. I am now in a place that is fanatical about GD&T and DFMA and design checking. I am enjoying it thoroughly.
Quite a few years ago, I designed an adjustable mirror that consisted of three screws and stiff springs. I positioned the screws 90° apart so that the adjustment would be fairly orthogonal. I moved the screws as far apart as I could, and I placed a note on the drawing describing the range and precision of the adjustment. The range is controlled by the solid height of the springs, and the compression that generates minimum acceptable clamping force. The precision is a function of screw pitch and separation. When the platform was redesigned for a new laser, someone moved the screws closer together and out of the orthogonal configuration. This made the mirror way more difficult to adjust. The resulting platform was left in production in that state for over ten years.
If they are stupid and they don't care, keep your resume up to date and network like crazy.
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JHG
RE: Shaft alignment spec on small machine
RE: Shaft alignment spec on small machine
Seems like resume time for all involved.
RE: Shaft alignment spec on small machine