I need a lesson on clearences

[ktm010]

I took apart my old #7 backhoe for some TLC. I was going to replace the brass bushings in the main frame measured 1.250 outside 1.000 inside seemed simple enough. orderd thru Mcmaster Carr, problem when I installed them they are to tight. I should have orderd thru JD so much trying to save some money. Next the lower assembly for the boom has some play, measured 1.014 I assume a fresh pin measures 1.000 is this excessive, the pin I have seems to move around quite a bit, and I noticed this when operating it. I'am not a mechanic, so any info would be great.

 

[Just Bill]

Hello
Even though .014 might be a little excessive, and .005 - .010 might be better.
I am not sure I would worry about it.
As long as the pins and bushings are round and not egg shaped, you might as well get a little longer run out of them.
Keep them greased and keep running them in my opinion.

 

[RoyJackson]

Did you actually install those 1.250" OD bushings? Then why do you think they're too tight? They should be a press fit, so don't think they're going to slip right in. Or, put the bushings in your freezer for a few hours then IMMEDIATELY install them.

BTW, Deere may have those bushing and pins made in metric sizes (rather then inch/pound measurement). That may account for the fitting problems you are experiencing (that is, replacing metric sized fittings with the nearest inch size equivalent). I don't have a backhoe, but since your tractor uses a #7, it's probably one of the Yanmar built machines like my 790. Most of the 790's fasteners and fittings are metric.

 

[sal64]

hello what happens with brass or bronze buss is when u press them in if the press is to tite the id shrinks and needs to be reamed out,mostly with that thin wall.

 

[ktm010]

The brass bushings go in with the same force used to take them out, problem is the 1" part will not go in after installed. The same goes for the larger main pin bushing 1.500" ID and 1.750" OD with new bushing installed I don't think I could even hammer it in. But they both slide on and fit nice and snug before installed. As for the boom pin, the pin fits nice into bushings on the boom the movement is in the frame piece as it rides on the pin.

 

[RoyJackson]

The brass bushings go in with the same force used to take them out, problem is the 1" part will not go in after installed. The same goes for the larger main pin bushing 1.500" ID and 1.750" OD with new bushing installed I don't think I could even hammer it in.

Then take sal64's advice and ream them out. You can probably rent a hand reamer at a local hardware store. Or, load the 'hoe on your truck and take it to a local machine shop. Those pins should have been a slip fit, IIRC (meaning you shouldn't have to drive them in or out).

 

[whistlepig]

.005" would be good clearance. .002"-.003" would be better. You can take a flapper sanding wheel on a pencil grinder and open the bushing up. Be gentle and only take small amounts at a time. Don't want to get the hole out of round.

 

[sal64]

i agree use a flapper sander ,i have oppened many id bores that way,but i use a piece of 1/2 rnd steel or use a 1/2 bolt hacksaw down center of the steel about 1 inch then get a piece of 220 grit paper back sand paper rip off 1 inch piece slide down the hacksaw slot rap it around the steel then put in your drill works very well.

 

[Farmerford]

Are you sure the old bushings are bronze rather than steel? The pressures on the bushings may exceed acceptable limits for bronze.

Commercially available thin wall bronze bushings are typically larger in both internal and external diameters than their nominal size. The bore into which the bushing is pressed is sized to give the desired inner diameter. Since the bushings are thin walled, they tend to shrink to fit the bore in which they are inserted. For example, a nominal 1.000" ID X 1.250" ID bushing with 0.125" thick walls might be (before installation) 1.002" ID X 1.252" OD. That's why it slides easily on a 1.000" rod. If pressed into a 1.251" bore, which is a 0.001" interference fit, the ID will shrink by about 0.001", giving a 0.001" running clearance, which is typical for oilite bronze bearings on a moderate speed shaft (electric motors, etc.).

But if the 1.252" OD oilite bushing is pressed into a 1.250" bore, then the ID shrinks by 0.002" to 1.000", which is a very tight fit on a true 1.000" OD pin.

Oilite bushing manufacturers recommend against sanding, drilling, or even reaming the installed bushing for various reasons. Sanding leaves grit in the pores, and drilling and reaming wipe the surfaces and close off the pores that carry oil to the bearing surface.

If the bushing is plain bronze rather than oilite, these procedures (sanding, drilling, reaming) should give acceptable results.

 

[whistlepig]

Machine shop rule of thumb for this type of running clearance is .001" per 1" of diameter. I.E. for a 1" diameter shaft .001" clearance would be acceptable. For a 6" diameter shaft .006" clearance would be acceptable.