bushing clearance

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liveaboard
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bushing clearance

Post by liveaboard »

Hi gang;
I wonder if anyone can tell me what clearance I should be trying to achieve for a 4" diameter bronze bushing.
width is 1"
The other bushing is 2" diameter and 1-1/2" long
Application is the front axle pivot of a 4 ton agricultural tractor. load over both bushings varies between 1 and 3 tons.
Grease is pumped in during periodic maintenance.
rods and housings have been cut.

I don't have a micrometer that big, so I'm forced to use a caliper [mitutoyo old fashioned vernier]
PLEASE DON'T TELL HAROLD!!
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warmstrong1955
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Re: bushing clearance

Post by warmstrong1955 »

I do lots of these.

For the the 4" & 2".....
You would want an average press fit, bushing to boss, of .003". (.001" to .005")

For the 4"....
Result is to obtain a pin to bushing clearance of .003" - .008" nominal when installed. Machine your bushing ID accordingly.

For the 2".....
The resulting fit should be .002" - .006" clearance after installation.

Use a 100% bushing close in to calculate the average fit to the pin OD.

Pretty forgiving....but I'm still not keen on the caliper.......
Harold will flog you later...... :wink:

Above are from standards we used at a couple of mining equipment manufacturers I worked for, as well as what we used for rebuilds & repairs on many jobs I was also on.
Standards were the same regardless of the material, other than 'plastics'.

Bill
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Harold_V
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Re: bushing clearance

Post by Harold_V »

One does with that is at hand, but, be aware--and this is the reason calipers are not a good choice: It is not at all uncommon for the inside and outside jaws of calipers (doesn't matter what type--vernier, dial or digital) to not measure the same. I returned a new 12" Mitutoyo that varied by .003", making them virtually worthless. The replacement was much better, and I own it to this day.

That in no way addresses the fact that the average user applies them in a less than acceptable method. Readings are often influenced by the desire to read the target dimension instead of the actual dimension. When in doubt, don't watch the reading---just pay attention to how the instrument is applied, so angles are correct. Trust your feel, and strive to make it repeatable.

Do keep in mind----fits like this used to be established with spring calipers. It can be done---it's just not as precise, although one's skill level increases with experience. That's one of the reasons bore gauges can be reliable.

Harold
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liveaboard
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Re: bushing clearance

Post by liveaboard »

Thanks a lot guys; I'll give it my best shot.
I know exactly what you mean about reading what one wants to read; observer's bias in the machine shop.
The press fit will be very tough, being at the limit of readability; I will be depending more on direct test feel of the parts between cuts. The material is expensive and I have enough for 1 bushing.
I do have a micrometer for the smaller bushing, but no bore gauge. I rely on the springed pin type ones, and read it with an external mic.
Yes, tricky. I take a lot of time but have been succeeding in making press fits.
Usually.
That means better than half the time. But not with a caliper!
Thanks for not flogging...
The tolerances warmstrong gave for the grease fit is a pretty wide range, I can do that.
f350ca
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Re: bushing clearance

Post by f350ca »

Bill not doubting the standards, but at .002 and .003 inch clearance will the bushings take grease? I've had trouble in that range.

Greg
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10KPete
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Re: bushing clearance

Post by 10KPete »

Don't forget the 'trick' of making plug and ring gages to use to check your sizes as you sneak up on them. Make a plug that just squeaks in the hole then use that to make a ring with the desired ID that will match the OD of the finished bushing. When the bushing squeaks into the ring it will have the desired press fit.

No calipers required!

Pete
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warmstrong1955
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Re: bushing clearance

Post by warmstrong1955 »

f350ca wrote:Bill not doubting the standards, but at .002 and .003 inch clearance will the bushings take grease? I've had trouble in that range.

Greg
If you have an actual .002" + clearance, and an aligned fit, yes....it will take grease. Been using this standard for 40 years.
This is assuming you have grease porting either located in the neutral axis, not the load side, or a center groove in the ID of the bushing.
Also assumes you have a very respectable measurement on the ID....as in grooves & peaks.

Less than .002", and it can it less than friendly. .001" and less....you will have problems, especially as the bushing gets wider.
.003" is the goal, which, when you consider that this standard is made with TG&P specs, and clearance is calculated with a pin diameter that is nominal, the pin diameter is actually less than nominal, that's easy to hit.
ie; TG&P spec for 4" dia is +.000"/-.003". Generally, it comes in at 3.999". / 3.9985"

Measuring with a caliper?
Better open the ID up a bit.....been there.....honed that..... :roll:

:)
Bill
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Harold_V
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Re: bushing clearance

Post by Harold_V »

liveaboard wrote:Thanks for not flogging...
That's counterproductive. I also feel it a requirement that the expected pitfalls be mentioned, perhaps with the idea that a part might not be scrapped.

I realize that none of us have every tool desired. I'm somewhat in that boat, as I specialized in small work, so I have no micrometers beyond 6". It's for that reason I acquired the 12" caliper mentioned. I also observe my own admonition to use them properly, but still have serious reservations in regards to the value of any readings so determined. I know they're not reliable---but as I said, one does with what one has at his/her disposal. It was not worth it to me to invest in large micrometers when they would sit idle well in excess of 99.99% of the time. Even my 2" micrometer saw limited use.

Harold
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10KPete
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Re: bushing clearance

Post by 10KPete »

Let's not forget that there is a difference between absolute measurement and relative measurement.

I use ID and OD micrometers and even though they are calibrated, truely, when I'm fitting a pin and bore I use the ID mic as though it were an adjustable gauge. I get the fit/feel in the bore then measure the ID mic with the OD mike to get a number. I use the same OD mike to measure the pin. So I only care about the difference, or clearance, not the absolute number.

There's lots of ways to avoid the "Are they calibrated exactly the same?" problem.... And I use those ways whenever possible 'cause I have learned the hard way....

Pete
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Harold_V
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Re: bushing clearance

Post by Harold_V »

10KPete wrote:Let's not forget that there is a difference between absolute measurement and relative measurement.

I use ID and OD micrometers and even though they are calibrated, truely, when I'm fitting a pin and bore I use the ID mic as though it were an adjustable gauge. I get the fit/feel in the bore then measure the ID mic with the OD mike to get a number. I use the same OD mike to measure the pin. So I only care about the difference, or clearance, not the absolute number.

There's lots of ways to avoid the "Are they calibrated exactly the same?" problem.... And I use those ways whenever possible 'cause I have learned the hard way....

Pete
Unfortunately, there are situations where that simply doesn't work. An example is working to a print, where sizes and tolerances are specific, and required to be held. In such a case, it is important for the (actual) size to be known, as you can't work to a dimension if it is unknown, and will succeed strictly by chance. Knowing where you are in regards to size, assuming you have the required skill and equipment to work accordingly, allows success by design.

In establishing fits for bushings, it's important for the inside diameter to be sized according to the amount of press intended. Thin walls will accommodate very little in the way of press fit without collapsing. That's true of even drill bushings, which are hardened through. Anything beyond a few tenths tends to collapse the bushing, often at the expense of a drill no longer fitting the hole. They really don't like that.

Harold
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10KPete
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Re: bushing clearance

Post by 10KPete »

Yes, of course when working to a drawing.....

Pete
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Patio
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Re: bushing clearance

Post by Patio »

liveaboard wrote: I do have a micrometer for the smaller bushing, but no bore gauge. I rely on the springed pin type ones, and read it with an external mic.
Yes, tricky. I take a lot of time but have been succeeding in making press fits.
Usually.
Using Spring gauges also called snap gauges, is an acceptable way, of taking a very accurate measurement. Even Harold endorses their use.
I do not look at my instruments when taking measurements. I concentrate on the feel, of setting the gauge, and then read it.
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