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 Q & A regarding fitting of components

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rjw
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PostSubject: Q & A regarding fitting of components   Fri Sep 18, 2009 11:24 am

Anyone care to share info on fitting of components? Such as how tight or loose a wheel or gear should be when it has to rotate on a shaft?

Example: An aluminum pulley needs to turn freely on a 1/2" diameter steel shaft. The bore of the pulley is 3/4" and I want to use a bronze bushing between the shaft and pulley.

Here are a few questions:

1) What size do I make the inside and outside of the bushing?
2) How does material selection affect the sizes that I choose
3) How about lubrication, heat, expansion?
4) How does speed and load affect all this?

Please feel free to answer any of the questions, add comments, or pose a new question of your own...and let's share some knowledge

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Cody
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PostSubject: Re: Q & A regarding fitting of components   Fri Sep 18, 2009 11:45 am

dont use bronze hahaha
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rjw
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PostSubject: Re: Q & A regarding fitting of components   Fri Sep 18, 2009 11:54 am

Cody Harris wrote:
dont use bronze hahaha

A good comment....care to add to this? as in why? or make other suggestions?

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PostSubject: Re: Q & A regarding fitting of components   Sat Sep 19, 2009 2:42 pm

Ive always felt bronze was to heavy, plus you can get to higher speeds with bearings
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PostSubject: Re: Q & A regarding fitting of components   Sat Sep 19, 2009 5:45 pm

This is what my dad had to say about this:

"In general there is very little clearance between precision fitting components. Look at the bearings and shafts in McMaster and you’ll see 4 decimal place tolerance dimensions. Usually it’s about 0.001 clearance for normal size bearings. For example I’m building something now at HVC where I bought s.s. ground shafting and it’s exactly 0.3750 diameter, and the heavy duty sleeve bearings that fit on it are exactly 0.3760 ID.

The speed and load are handled by the P and V parameters used in bearing selection charts.

The surface finish and hardness of the parts plays a role here too, shafts must be pretty hard and have a ground finish. Bushings need to be softer than the shaft so that they wear out and not the shaft. The shaft needs to be round too, that’s why precision shafts are ground not lathe turned for their final diameter.

The lubrication one uses is sort of a common sense thing. You use grease for heavy loads and moderate to low speeds. You use oil for lighter loads and higher speeds. Look in McMaster at the bearing blocks and pillow blocks. Look at the ones with grease fittings, look at their speed rating. Look at the ones that don’t have grease fittings but instead have an oil hole, again look at the speed ratings. This will give you a ball park idea.

Material selection comes into play in any significant way only if the wrong materials are being used. If one sticks with conventional combinations of materials it all works and you don’t have to think about it much. Hardened and precision ground shafts made of steel or s.s. always work fine with sleeve bearings made from sintered bronze and other sintered alloys, frelon, PTFE, and other types of polymer blend bearings. Most bearings have a specified shaft hardness and finish that they require for proper use.

Lastly it depends on how long the bearing is supposed to last. If it’s just a few minutes of combat that’s different than something that has to run a few thousand hours a year.

Another factor is length to diameter ratio. 1:1 is a minimum, 2:1 is better (length : diameter) but it depends on what you’re doing with the bearing."

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rjw
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PostSubject: Re: Q & A regarding fitting of components   Sun Sep 20, 2009 12:07 pm

Cody Harris wrote:
Ive always felt bronze was to heavy, plus you can get to higher speeds with bearings

This brings up more food for thought (Dan has touched on a lot of this as well)

In building combat bots with weight limits, we are always looking at what components weigh.

In my simple example above, you might find that a simple thin wall bronze bushing might weigh less than a bearing(s).

Some other factors to consider are: (please note that these are very general statements)

1) Speed....rolling elements such as bearings are generally better for high speed applications.

2) Load....bushings will generally support MORE radial moa than bearings. Ball bearings are the worst, in that they only have POINT contact...i.e. the load is supported by contact between the balls and the iner and outer races of the bearing. Needle bearings are a better choice if radial loads are high. Bushings have the most surface contact and can support a large radial load. Other bearings such as tapered roller, cylindrical, etc. could also be considered, but they also have thier limitations.

On the other hand, ball bearings can resist some AXIAL loading (about 1/3 of their raial load rating), while needle bearings provide NO axial load resistance. Bushings can be designed to support axial load. Tapered roller and angular contact bearings provide both raial and axial support.

3) Friction...rolling elements such as bearings create much less friction than sliding components such as bushings.

4) Lubrication...In most combat robot applications, grease lubrication is desirable. Some greases can also be used at fairly high speeds. In extreme speed applications, oil lubrication is preferred.

5) Component life and service factor...Items 1 thru 4 above all play a role in determining how long components will last.

Shock load is often either overlooked or underestimated. For example, A ball bearing of a certain size might be a good choice for a simple pulley rotating on a shaft, that won't see much shock loading, but in the case of a wheel that might hit the ground from 5 or 10 feet in the air, it may not hold up. When choosing components, we need to add a service factor to the expected load ratings.

6) Machining of components...For longest life and best performance, machining accuracy of shafts, housings, etc. must be considered. Bearings generally require higher machining accuracy than bushings.

More to follow.....

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rjw
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PostSubject: Types of fits   Sun Sep 20, 2009 12:51 pm

When fitting components such as a gear , pulley or wheel to a shaft, consideration must be given to the type of fit that is best suited to the application.

Fits can generally be divided into PRESS fits and RUNNING fits.

These can be further divided into loose and tight (press or running) fits. There are also fits thatfall in between these.

For example, a press fit can be the tightest fit that can be assembled by hand, using a press or even heat to expand and contract one component onto another.

running fits can vary from a tight accurate fit to a very loose, high load fit.

The tolerance of shaft diameters to mounted member bore can vary greatly and is also epenant on size.

Can someone with a Machinery handbook list the tolerances for these? and typical applications?

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