When line boring the boom end of a excavator, bucket arms of a loader, or basically any connection which uses press fit bushings, what is the rule of thumb for finish ID bore size? I’ve always used the thousandths and a half undersized per inch of bushing OD. So if the bushing is 3.000” outside diameter, you’d machine the bore to 2.997” - 2.9955”. Does anyone know if this is correct? I’ve measured bores which only need bushing replacement without line boring and found a range of size variations. Sometimes is a few thousandths and other times it’ll be 10-12 thousandths undersized. I’d like to ask for anyone else’s thoughts and experiences. I’m assuming a bore 10 thousandths undersized would cause distortion to the inside diameter of a bushing when pressed in under that type of under sizing. Thank you!!
What bush material are you talking about.? Steel, phospor-bronze, etc.?
For personal preference I like to freeze the type of linkage bushes you are talking about in liquid nitrogen and also heat the area around the bore, then they generally push into place quite easily and all issues of distortion fall away.
I use the freeze/heat method as well. Pressing just takes too much time, requires expensive equipment and honestly gets pretty sketchy with 25+ ton on a piece of Acme rod. Always seems to pick up metal from the bore on the way in too and that can lead to it getting jammed part way into the bore.
I've seen bushes pressed into excavator arms that lasted a day then started spinning. To fix the arms had to be line bored. The heat/shrink method is best.
I think i read here about measuring the difference between pin od, the bush id and making sure the interfeerence between the bush amd bore is less as the bush id will srink by the interference amount and you could end up with a locked up pin. There are people here with miles more knowledge on the subject.
Where is Rayf ? AWOL almost two years. I think he was the most knowledgeable line-borer that ever posted on HEF.
.003 is a pretty tight press fit for some stuff, if the metal going into the hole is steel or similar and the part with the hole is aluminum, and the whole thing is subject to heat then the press fit is in the .007 or so range some times a bit more. Even a pin that is the same diameter as the hole is a press fit. I have run machine tools where we held plus minus .0002 for holes in a certain part. Yeah can you imagine scrapping a part if it is out by .0001 of an inch? I suppose some fuel injection pumps of old where pretty fussy on the tolerances as well as hydraulic tappets.
.001/1 inch diameter undersize is a standard press fit, .002 is a heavy press fit. For thin wall bushings, whatever you have for interference is how much collapse you will get in the bore. A shrink fit will have more holding force than a press fit if they have the same amount of interference.
This is the most often asked question when boring holes for bearings and the easiest to answer. Firstly toss rule of thumb theories out the window. The manufacturer has there own idea how much interference they want on a bearing. When they design it they build into it the interference and the fitted pin to bearing clearance. Bores are almost always bored to nominal size. A typical instance with a Cat bearing would measure 4.004. Cut the bore 4.000. An Hitachi bearing would typically measure 230.4mm Cut the bore 230.00. If you can access specs you will see besides the size a tolerance. It could be H8. Look up your black book and check H8 for that diameter. It may be .003 so your tolerance is minus 0.0 to maybe .003 plus.
Hi Ray ! Nice to see you back and posting.