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Shrink Fit Failure

Shrink Fit Failure

Shrink Fit Failure

(OP)
I am currently designing an assembly that requires a sleeve to be shrink fitted onto a potentiometer shaft. I am having issues with analyzing the stresses caused by the fit. I have tried using an online calculator, however no matter what I do I always seem to have stress levels that are too high. I have summarized the information below.

Material
Hub Material: 360 Brass Alloy
Shaft Material: 303 Stainless Steel

Dimensions
Shaft Dimensions:
Maximum Shaft Diameter: 0.1248"
Minimum Shaft Diameter: 0.1245"
Length of Shaft: 0.313"

Hub Dimensions:
Maximum Inside Diameter: 0.1244"
Minimum Inside Diameter: 0.1242"
Outside Diameter: 0.25"
Length of Hub: 0.4375"

Fit Specifications:
Type of Fit: Light Drive Fit [FN1]
Maximum Interference: 0.0006"
Minimum Interference: 0.0001"
Engagement Length: 0.1309"


If I need to specify any more information, let me know. Any help would be appreciated, thank you.

RE: Shrink Fit Failure

(OP)
Thanks for the help. One quick question though, what exactly is the δ symbol? Is that the symbol for the amount of interference?


RE: Shrink Fit Failure

If your seeing that symbol on the tribology site I don't know because my comp won't open the calculator sorry

RE: Shrink Fit Failure

I am always really hesitant to use online calculators. I remember one that converted 1 ton to 64400lbs (2000*32.2). Remember that they may or may not be anywhere near close. That being said, this one looks pretty good.

The symbol is a small "delta", and as best as I can tell, denotes the interference in your fit.

A shrink fit is a function of thermal expansion and elasticity (so long as you stay in the elastic range), and generally need not be overly complicated. The indeterminate solving assumption is (generally) that the stress at the interference faces is the same for the outer and inner components (i.e. the inner pushes out as hard as the outer pushes in, given identical dimensions). A usual mistake is the assumption of identical deformation between the parts. This is not a correct assumption.

Machinery Handbook and other design books (Norton, Shigley, Roark) layout typical methods for designing shrink fits.

"If it has been done... there is a better way."

RE: Shrink Fit Failure

δ is noted on the website: "The value of the diametrical interference is typically about δ/d=0.001. The calculator is based on elastic deformation (Lame's equation), i.e. the stresses should be smaller than the elastic limit Rp0.2 of the elements."

I think I would just stop that sentence at 'diametrical interference.' as the value they show for δ is (d*.001)

Based on the online calculator, it looks like you need
nominal dimensions,
amount of interference,
Poisson's ratio shaft and hub,
elastic modulus shaft and hub,
elastic limit shaft and hub and
friction coefficient.

From this you get
how much torque the joint can carry,
whether the parts fail due to plasticity, and
how much axial load will be required to install/remove, and
the side knowledge of what the contact pressure is between the bushing and the shaft.

RE: Shrink Fit Failure

one thousandth of the diameter is a good place to start, it will be the correct order of magnitude (at least that is my experience when working with two steel components).

RE: Shrink Fit Failure

(OP)
All right, after playing around with the calculator fit, I have found that polycarbonate (40% glass filled) would be a suitable material for hub, with a FN4 fit. Would using that polycarbonate be a good idea? I'm not too familiar with the material.

RE: Shrink Fit Failure

(OP)
Hello DesertFox,

Yes I started with both metallic options, however fiddling with the online calculator led me to plastic material for the hub, since metallic material caused stresses that were large. I am now onto plastic materials, and am trying to find the right material. I am designing a gearing assembly, that would require the hub to be shrink fitted onto a potentiometer shaft. The hub will have a knurled exterior, and be press fitted onto a gear. I would attach the potentiometer shaft to the gear directly, however I cannot modify the shaft (in order to knurl or have a key or spline), and cannot find a gear with a bore size that would have the right dimensions in order to permit a shrink fit. I need the fit to be able to withstand a load of 40 oz-in.

RE: Shrink Fit Failure

Hi CodeRed7

Well you can have polycarbonate gears, do a search and you'll find loads of them.
You could try and buy a blank gear in Nylon or polycarbonate and machine in the bore for the shaft.
What is the environment for these gears?

RE: Shrink Fit Failure



What characteristic of the original selection seemed to be insurmountable?

One option not mentioned is using a shaft locking compound to tie the metal gear to the shaft without a press fit.
Another option is to have the gear supplier alter the bore to match - in many cases bores are purposely small to allow this option so the supplier can have in-stock gears requiring only this operation.

RE: Shrink Fit Failure

(OP)
Desertfox
The gearing assembly will be attached to a balancing valve that will be fitted on pipes in the mechanical room of a building. So the temperature would most likely be around 25-30 degrees Celsius.

3DDave
If I were to use brass as the material for the hub, the shrink fit would result in a stress that is higher than its yield strength. Playing around with the numbers in the calculator, I found that I needed a material with a lower modulus of elasticity, which made sense because lower modulus' cause lower stress when strained. I decided to go with plastic as it had a lower modulus.

The shaft locking option and bore altering option are both possibilities. My only concern with both would be the price for both. Would you happen to know generally which would be more expensive? I will do the research anyways, but would like to know a general opinion.

Thanks for all the help :)

RE: Shrink Fit Failure

Hi CodeRed7

Normally you would use a key to fit hub and shaft together but that's not an option, so I can see only machining the gear bore to suit shaft. Machining a matching bore is done all the time and that's the way I would go.
One thing to mention what coefficient of friction did you use, I imagine that the friction coefficient is lower for plastic on metal; than metal to metal, if so it means the interference must increase.

RE: Shrink Fit Failure

(OP)
Hi DesertFox,

I used 0.35 as the coefficient of friction between steel and brass, and used 0.39 for the polycarbonate. That is the opposite of what you mentioned, but my sources could be inaccurate. I will double check, but would you happen to know of a reliable website?

RE: Shrink Fit Failure

hi CodeRed7

I found similar figures to yours, I was surprised though.
just looking at polycarbonate and comparing it with brass it appears that brass has a better yield stress than the polycarbonate (http://www.makeitfrom.com/material-data/?for=Polyc...) according to the figures I'm seeing.
http://www.efunda.com/materials/common_matl/common....

According to my rough calculation with an interference of 0.0006" and using steel and polycarbonate you will transmit a maximum torque of 9.41oz-in and that's nowhere near 42oz-in.
I'll check my figures again, is there any chance you can post your calculations?

RE: Shrink Fit Failure

"I need the fit to be able to withstand a load of 40 oz-in."

What kind of potentiometer requires that level of drive? Is it an inertial transient load?

Give a Google a search for "Shaft Locking Compound" or "Shaft Retaining Compound"

RE: Shrink Fit Failure

hi CodeRed7

I should of added in my previous post that you should design the system from the external load components first and then work backwards towards the driver, in this case a motor, but it needs to be sized based on the external load requirements and then you don't end up with a motor that's under powered.

RE: Shrink Fit Failure

(OP)
3DDave:

I misunderstood how the torque will applied to the load, and as a matter of fact only need the fit to withstand a load of just greater than 0.7 oz-in, so most plastics would work in this case. But yes, I will google that right away.


Desert Fox:

And yes, brass has a higher yield strength, but also has a larger modulus of elasticity, which results in larger stresses when the material is strained. I used the calculator mentioned above, and I will attach both the results for the brass and for the plastic I used, which I have changed now to nylon 6,6 due to the lower torque requirement. It's not important now, but I used 40% Glass filled polycarbonate, which has different mechanical properties. I'm sure your calculations were correct, as I got similar results when I used normal polycarbonate. Also, the motor has already been selected, and can already generate enough torque to drive the loads.

The calculations are all in metric, and converted all my measurements to metric as such. For reference, 0.7 oz-in is equal to 0.005 Nm, and the yield strength of Nylon and Brass are 63 MPa and 125 MPa respectively. As shown by the calculations, the stresses for brass are larger than 125, which is why it was ruled out. For the nylon calculations, I used interferences of 0.0001" and 0.00006" (since those are my interference limits) to make sure the allowed stresses and torques were within their limits for both.

I will double check all calculations by hand when the material is finalized, but am using the calculator for a general guideline.

Attachment Order
1) Brass
2) Nylon - 0.0006" Interference
3) Nylon - 0.0001" Interference

RE: Shrink Fit Failure

(OP)
Also I was wondering, would nylon be able to be machined to those tolerances? If not, are there any other plastics that I can use, or would I have to go with a different fit?

RE: Shrink Fit Failure

Hi CodeRed7

If possible I would increase the interference provided the nylon stresses don't get to high and doing that would give you more margin for error.

desertfox

RE: Shrink Fit Failure

(OP)
Hi DesertFox,

Allright that makes sense, there are standard fits (with standard interferences) that are posted on the ANSI guidelines. Should I follow those standard fits or would I be able to design my own?

RE: Shrink Fit Failure

hi CodeRed7

No reason why you can't do a fit of your own.

RE: Shrink Fit Failure

(OP)
DesertFox,

Thanks again for everything. Right now I'm looking into the shaft retaining compound that 3D Dave mentioned as it removes the need for the extra sleeve. If this does not turn out, I will go back to the shrink fit with the sleeve.

RE: Shrink Fit Failure

Hi

your welcome!

RE: Shrink Fit Failure

1) what's wrong with a press fit yielding one of the elements ? (it seems this is why brass was de-selected) actually, isn't this a good thing 9'cause it'll increase the torque that the joint can react (ok, torque isn't a big design element here).

2) i don't know your application, but be careful choosing materials. i would caution changing from brass to plastic to suit one element of the design. why did you pick brass to star with ??

Quando Omni Flunkus Moritati

RE: Shrink Fit Failure

Plastics behave differently than metals. You will not be able to maintain a "shrink fit" between a plastic component and a metal component over time. The plastic component, whether it's nylon or polycarbonate, will creep and the interference fit will be lost. If you want to make a reliable connection between a plastic part and a metal part, you must use some type of adhesive bond.

RE: Shrink Fit Failure

There are a few issues not covered so far:

If you use an adhesive to bond shaft and hub, you may have problems with the adhesive wicking into the potentiometer's internal bearing.

If you use a press fit to bond shaft and hub, you may dislodge the tiny snap ring adjacent the potentiometer's internal bearing and damage the potentiometer.

Both potential problems go away if you buy the potentiometer with a flatted shaft and push on a plastic hub with a molded D hole. That also allows you to mechanically index the plastic hub's features to the potentiometer's wiper with manufacturing controlled geometry, and avoid the need to manually angularly adjust the wiper position to that of the sensed mechanism during assembly.



Mike Halloran
Pembroke Pines, FL, USA

RE: Shrink Fit Failure

(OP)
Hi All,

Rb1957
1) It notes on the website and I have heard in other discussions to compare the stresses that result from the calculations to the yield strength. My assumption is that since the material is trying to fully return to its original shape, this causes the force required to maintain the fit. If the material is yielded, the force caused by the interference will be decreased since the material is not trying to fully return to its original shape. Either way, the stresses that I found when using brass exceeded its ultimate tensile strength, so the material would fail either way.

2) It choose brass since multiple other components in the assembly are made of brass, but these will be under larger torque requirements (100 oz-in), whereas this fit will be under much smaller (around 0.7 oz-in).


Tbuelna
Thank you, I will keep that in mind, I will most likely go with using the adhesive anyways.

MikeHallorn
Unfortunately, I have already ordered the potentiometer. Either way, I don't think I was able to find a potentiometer that met my specifications that had a flatted shaft. In addition, I couldn't find a stock gear with a D hub, this would most likely have to be a custom gear would be more expensive. Would you know of any way to avoid the adhesive wicking into the potentiometer's internal bearing?

RE: Shrink Fit Failure

(OP)
Hey guys,

I was wondering, are there any issues with using a clamp with a clamp hub gear? This seems to be a feasible option that has not been mentioned yet, and was wondering what the limitations of this option were.

Thanks.

RE: Shrink Fit Failure

That should work.
Be sure to leave the clamp accessible for adjustment.
... and to slot the end of the pot shaft for a screwdriver, or let it project beyond the clamp so you can grip it for adjustment.

Mike Halloran
Pembroke Pines, FL, USA

RE: Shrink Fit Failure

(OP)
I can leave the clamp accessible for adjustment, however most of the clamp hubs I can find are longer than the potentiometer shaft, and the shaft isn't slotted. Are there any other solutions? Just to clarify, I'm using a gear with a clamp hub, a potentiometer, and a separate clamp. If you want to look at the products I have listed them below.


Potentiometer: http://www.bourns.com/data/global/pdfs/3549.pdf
Gear: http://sdp-si.com/ss/pdf/79001063.PDF
Clamp :http://sdp-si.com/ss/pdf/81001242.pdf (Model No. S3501Y-C065)

I was wondering, why exactly do I need to able to adjust the clamp? My application is permanent, so I would not require disassembly. Would the reason be that it can become loose after a certain amount of time?

RE: Shrink Fit Failure

If I haven't misunderstood something, you're using the potentiometer in a control system, to measure the position of a valve.

At some point during assembly, someone has to physically move the valve to one extreme of its motion, and move the potentiometer to the corresponding extreme of its motion, and tighten the clamp, so that from then on, a particular position of the potentiometer's wiper corresponds to the analogous position of the valve's rotating parts.

Potentiometers typically ship with a rather long shaft, that may even be annealed for easier machining. It's fairly common to clamp the distal end of the shaft in a milling machine, mill a flat on the shaft without applying any force to the potentiometer itself, then cut the shaft to a desired length, and optionally slot the shaft. In that way, the machinist's setup process defines a controlled relation between the flat and the pot wiper. Said relationship will be documented in a 'make from' drawing. Once that relationship is established, further need for adjustment can be eliminated with D holes, and you don't have to pay servo techs for simple assembly operations.

You have chosen to eliminate the shaft machining process step, so the angular relationship between valve and pot wiper must be established at assembly time. Hence the clamp. A tiny drop of green Loctite on the clamp screws after adjustment would be a good idea.

Mike Halloran
Pembroke Pines, FL, USA

RE: Shrink Fit Failure

If this simply connecting the input shaft of a rotary pot to a valve shaft, then you probably want some sort of coupling that will isolate any forces being transmitted other than pure torsion. No bending or thrust. There are shaft couplings designed precisely for this purpose. Most rotary pots are not designed to accommodate any radial or thrust loads, so it probably not be a good idea to use a rigid shaft coupling.

RE: Shrink Fit Failure

(OP)
MikeHalloran
Yes, the application is correct, but would I not be able to adjust both the valve and potentiometer before clamping in order to achieve the angular relationship, and then apply the clamp, while being careful to not move either the valve or potentiometer? Would I still need to slot the pot shaft or have it project beyond the clamp?

Tbuelna
I forgot to mention a motor will be attached that will drive the system. The valve shaft will take most of the load, while the potentiometer will experience very little. Would I still need to look for a different shaft coupling?

RE: Shrink Fit Failure

We can't properly comment on particular features of your design, because we have seen exactly none of it.

We can suggest, however, that some sort of coupling (or flexible anchor) of a potentiometer might be a good idea, because their bearings are usually, well, trivial, and the tolerances and alignments in a typical valve actuator are not precise enough to not damage a 1/8" shaft with any sort of rigid connection.

Speaking of bearings, there really aren't any in a typical valve. If you are relying on the valve shaft to resist gear separating forces, and the valve manufacturer has not blessed your application, you may face serious warranty costs.

Mike Halloran
Pembroke Pines, FL, USA

RE: Shrink Fit Failure

(OP)
Hi Mike,

I have attached the basic design of my assembly. In my assembly, a coupler connects the motor and the valve, which transfers most of the torque. Attached to the coupler is a gear, which connects to another gear and then to a potentiometer. My idea for assembly would be similar to mentioned before. The potentiometer and the motor would initially be mounted. One of the gears would be press fit onto the motor valve coupler over the knurl, and then the coupler (with the gear attached) would be press fit onto the motor shaft (also over the knurl). The potentiometer would be manually adjusted to its central angular position, and then a clamp would be placed over the hub of the gear. This gear would be moved in alignment with the other gear and the potentiometer shaft, and then clamped. Finally, the valve will be adjusted to its central angular position, and the valve shaft will be slid into the motor valve coupler and mounted. This seems like a feasible idea, am I missing anything that would require slotting the pot shaft or having it extend beyond the clamp?

Also, are the problems you mentioned still apparent in this assembly? Would the fact that the potentiometer is not directly connected to the valve shaft maybe solve some of these issues?

RE: Shrink Fit Failure

Knurls are for providing traction on hand contact surfaces.
They are not real good for mounting gears,
nor for transferring torque.

Your structure is commendably minimalist, but it's rather delicate. If it's possible to bend something with bare hands so that, e.g., the gears dont't mesh right, some end user or random twiddler will do that, and you'll pay for a warranty call.

Mike Halloran
Pembroke Pines, FL, USA

RE: Shrink Fit Failure

Looking at your sketch, the main thing I would recommend is making sure your gear mesh has adequate backlash to prevent binding. Mesh binding would produce excessive radial loads on the pot shaft. Based on what I can see from the sketch, it looks like your mounting arrangement and bracket may not provide optimum shaft alignment and stiffness.

RE: Shrink Fit Failure

(OP)
Mike
I can also use the clamp to attach the coupler to the gear, so I will look into that option. And understandable, this is just the basic design for now, I will make sure that the assembly cannot be accessed by any random user.

Tbuelna
All right, I will look into backlash in order to reduce the radial loads, thank you.

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