Semi Rigid Bearing Housing - Feature Size and Form 2

[lwealing]

I have a question about a thin walled aluminum bearing housing (semi rigid?) in regards to feature size and form.

My application requires a tight tolerance for the bearing housing bore in which a deep groove radial ball bearing will be installed via a press fit. These bearings are custom made with special internal geometry for our specific application. Likewise, the housing size and tolerance has also been designed specifically for the application and the custom bearing. Because of the tight size tolerance for the housing, we are struggling to meet the Rule #1 principle and still be cost effective for my application.

In this example, we have two bearing housing bores on each side (horizontally opposed) with the same size, location, and form tolerances specified. Each bearing housing bore is identified as Datum A and Datum B respectively.

Details:
Bearing Housing ID Feature Size & Tolerance: ∅30+/-0.01
Bearing Housing ID Feature Position: [POSITION][∅0.1][A-B]
Bearing Housing ID Feature Form Control: [CYLINDRICITY][0.005]
*ALL DIM ARE MM*

Geometric Dimensioning and Tolerancing Standard: ASME Y14.5M-1994 (ASME Y14.5-2009 can be used if justified)

Currently we our suppliers are capable (Ppk>1.33) of making the feature within the size tolerance using the least squares method. However, when Cylindricity (form) is checked, we can clearly see that the suppliers are only capable to around 0.02. This also shows the same violation of Rule #1 when the minimum circumscribed cylinder and maximum inscribed cylinder is reported.

During development testing and evaluation combined with warranty data from systems in operation, the data shows that the current parts my company is receiving and selling are meeting customer needs. Therefore, as the designer, it is my responsibility to document the part's requirement to satisfy the customer need. I believe this evidence clearly indicates that the current specification is too restrictive.

I would like a way (as legal and standard as possible) to decouple the size and the form while still maintaining orientation of the feature.

I have searched somewhat extensively in this forum about this situation and have found a few useful threads:

http://www.eng-tips.com/viewthread.cfm?qid=223257

http://www.eng-tips.com/viewthread.cfm?qid=320036

http://www.eng-tips.com/viewthread.cfm?qid=324500

http://www.eng-tips.com/viewthread.cfm?qid=353421

In the above threads, you can see several ways that this might be accomplished. However, none are clearly straight forward according to the ASME Y14.5M-1994 or even the ASME Y14.5-2009 standards. Here are a few options that I can see might work. I would like some feedback about which option might suit my situation the best.

OPTION 1: Per ASME Y14.5M-1994
Use a combination of average diameter and form control to purposely violate Rule #1 per section 6.8.3 AVERAGE DIAMETER and as shown in FIG. 6-53. However, since this bearing housing is semi rigid (not flexible, but still can be deformed somewhat), do not use the Free State Variation symbol (F) in the FCF of the form control for Cylindricity. This is because the rest of the part is assumed to be measured in the free state and I do not have any constraint requirements for measurement of the bearing housing.

In this case does the size need to be expressed using the limit dimensioning technique as shown in FIG. 6-53 or is it possible to also use plus minus tolerancing for this case? For our use, specifying a target for feature size is useful for communication, but also calculating capability.

OPTION 2: Per ASME Y14.5-2009
Use the Independency symbol (I) per section 3.3.24 combined with using the text AVG on the feature's size specification. Then specify the form tolerance independent of Rule #1 as needed.

OPTION 3: Per ASME Y14.5M-1994
Use a combination of size tolerance and form tolerance that violates Rule #1 as needed, but use a note under the feature's specification or a flag note in the drawing to communicate that Rule #1 does not apply to denoted feature.

There might be other options available that I am not thinking of as well. Any suggestions or feedback would be helpful.

Thanks.

 

[MikeHalloran]

I believe this evidence clearly indicates that the current specification is too restrictive.

No such evidence is visible here.


Given that you are pressing bearing races into a thinwalled aluminum housing, the tolerances you apply to the machining process and to inspection of unstressed parts in the free state may be irrelevant. Do you monitor the press force? Is the press force the same on reassembly after disassembly? If plastic deformation is invovlved in assembly, then you may not know the conditions seen by the bearings in service. In particular, you probably don't know how large a margin you have in service life; you only know that it's adequate.

I'd advise a lot of caution about messing with a process that's working.
If it's working, why ARE you messing with it?


You are asking very detailed questions about a print that we can't see. At least meet us halfway with a redacted version.


Mike Halloran
Pembroke Pines, FL, USA

 

[lwealing]

Mike,

Thanks for your reply. I think I can add some answers to a couple of questions you posed here.

Do you monitor the press force?

Yes, we do monitor press force during assembly.

Is the press force the same on reassembly after disassembly?

Press force during reassembly is very similar (single digit percentage difference) to that measured during initial assembly.

The assembly is designed such that these cartridge bearings are easily replaced for service. We currently have several years of data that is consistently demonstrating that we are exceeding our customer's expectations for bearing service life. In most cases (except for extreme environments) we show that our bearing service life is more than 2X the customer requirements with current assembly components and requirements.

My goal here is to better document our current machining capabilities within the the rules of ASME Y14.5. We have agreement that our current capability is consistently meeting customer needs. There is no proposal to change the process, especially since engineering does not dictate the process, only more clearly document the requirements.

The question is how do we document our current capabilities within the rules?

Sorry I do not have a redacted specification available at this time. However I can add one as early as 02/24/2014. I agree this would help the discussion.

Thanks for your feedback. I hope my answers provided here can help shed some additional light on this topic.

 

[MikeHalloran]

Further comments:

2X in service life is NOT a big margin, especially if you had to cherrypick data to get there.

The purpose of a drawing is not to document your process capability.
The purpose of a drawing is to document the worst POS you will accept.






Mike Halloran
Pembroke Pines, FL, USA

 

[lwealing]

Sorry for the late timing, but had some time at work to put together this simple sketch. Please refer to the attachment.

 

[MikeHalloran]

I don't know if any GD&T type standard says it's kosher, but using [A-B] as a reference for true position of [A] and looks hinky to me.

Calling the OD [C] and using that for a reference sounds better to me.

Mike Halloran
Pembroke Pines, FL, USA

 

[ewh]

See Y14.5-2009 ¶ 4.12.4. The related Fig. depicts a runout condition, but the text does not limit it.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[Enginerd9]

Just a thought, why not make it a single thru bore with a spacer in between each bearing? Or a thru bore with retention rings as stops? Would it not be easier to control cylindricity and concentricity if the ends were made in the same operation? And wouldn't the bearings have a higher likelihood of maintaining axial alignment with a single operation bore?

Experience: accumulated knowledge over time.

Talent: the ability to use experience.

Which is more valuable?

 

[Enginerd9]

Here's another question: would I be off the mark to call the Dia. 32mm my Datum A, with an end as Datum B, then call the bore diameters concentric to A, true position to A and B?

And doesn't cylindricity imply roundness and axial alignment? So I'd reference that off of my Datum A as well, though it may be slightly redundant?

Just curious, I actually have a very similar part. Thanks!

Experience: accumulated knowledge over time.

Talent: the ability to use experience.

Which is more valuable?

 

[greenimi]

I am not a GD and T expert, but I would say that the positional callouts to A-B looks okay with me (of course if they are functional)
One small observation: please align datum feature symbol with size dimension (“A” symbol should be in line with the arrow for the size). The same thing valid for datum feature B symbol.

 

[Enginerd9]

Ah, yes. Cylindricity has no datum and implies circularity and straightness with all cross sections simultaneously. So I retract that particular question.

http://www.tec-ease.com/gdt-tips-view.php?q=96

http://www.efunda.com/designstandards/gdt/cylindricalspecs.cfm

Experience: accumulated knowledge over time.

Talent: the ability to use experience.

Which is more valuable?

 

[lwealing]

Thanks for everyone's input thus far.

However what I really wanted to stay focused on is the topic of independence of size and form when needed to override Rule #1.

Let's try to stay on track and focus on this topic.

 

[3DDave]

The problem as described is not solvable. The reason is that there are too many variations and not enough limitations to relate potential geometry to operating life of the bearings, which is the overall goal.

There certainly aren't any methods within the Dimensioning and Tolerancing described in the Y14.5 series that will solve this.

As a practical matter, the press fit into a thin wall is controlled by the wall thickness and the ID circumference. Changing to circumference will eliminate the need for a 'least-squares' evaluation of diameters. The bearing orientation is guided by side contact with the bottom of the bore, which should be a primary control. If you apply a position tolerance, RFS, to the first 10% of the bore near the side contact surface, that should provide enough coaxiality control from bearing to bearing. You'll need a note that limits wall thickness variation to some amount to prevent the bearings from being steered off course.

"Let's try to stay on track and focus on this topic." This is rude. The people here don't work for you.

 

[lwealing]

3DDave,

Thanks for your input - I never have use circumference in this manner before - but it is an interesting idea.

Also, I never meant to sounds rude at all. I apologize if it came across that way. I know many of the other contributors are trying to help, but in this case I was looking to keep the dicussion focused the use case rather than a discussion about composite datums and the like. I hope that you and others can understand. I do appreciate everyone's contributions. A big "Thank You" goes out to all the individuals who have posted thus far.

 

[Enginerd9]

Well, like I said... I have a similar part. We were having issues with bearing life and function, and discovered that holding true position and cylindricity on each bore was the cause. The fabricator would have to bore one end, then remove the part from the chuck, spin it, and bore the other end. You lose a lot of control that way, even in the best of set-ups.

So, to stay on topic, if you want a better performing part relationship, you can make it a single bore through with either a spacer or retaining ring grooves like I did. My results were very favorable... we produced a part which was easier and cheaper to make, held tolerances better, and performed better. The bearing noise was eliminated, and we haven't seen a cyclical failure yet since now our bearings are better aligned axially.

So, why not address the root cause rather than trying to re-skin the problem to make it prettier?

Experience: accumulated knowledge over time.

Talent: the ability to use experience.

Which is more valuable?

 

[lwealing]

Enginerd9,

Thanks for sharing those details about your prior experience. Really helpful. Would you have any reservations about sharing some information about size tolerance, Position, and Cylindricity requirements you had for your application?

In our application, we do use what we call "spacer tubes" similar to what you are suggesting for your application. So I do have experience using a similar setup to what you are describing. However this does come with a weight penalty for this type of solution. Believe it or not, the products that I design are sensitive to single digit grams and every one counts (think about something similar to the racing world). So I need to balance cost, complexity, robustness, as well as overall weight. Weight is always one of the highest priority customer requirements. In addition, this solution works well for bearings with ODs of the same size. I made them the same size for this simplified case, however we also have cases where the OD is not the same diameter. This is one of the reasons we make OD bearing stops directly into the housing.

Currently our supplier can do both bearing bores in a single setup. Position is bang on as you might imagine (∅0.05 or lower), just they struggle to meet the Cylindricity requirement of 0.005 mm.

Any info you can share might be very helpful for my situation. Thanks in advance.

 

[MikeHalloran]

Assuming your supplier does not have a two-spindle machine, doing both bores in one setup suggests that one bore is being done with a boring bar, 'back boring' and 'back facing'.

Boring bars are not infinitely rigid, and often vibrate, alternating between dragging and cutting, when cantilevered substantially, as here. The need for back boring and especially back facing limits the size of the bar that can be used.

Specifiying a complete straight through bore with a couple of snap ring grooves allows use of a much larger bar for cutting the through bore, likely reducing your cylindricity problems and saving weight.

You could also buy your custom bearings with an external snap ring or a flange on their OD, saving the cost of cutting snap ring grooves in the housing bore.




Mike Halloran
Pembroke Pines, FL, USA