MolaKule
Staff member
OK, we still see these terms commonly misused and interchanged even today.
1. Define a Mechanical Tolerance.
2. Define a Mechanical Clearance
1. Define a Mechanical Tolerance.
2. Define a Mechanical Clearance
Tolerances and Clearances
- Thread starter MolaKule
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1. Tolerance. Allowed +- range of the dimensions of a part or clearance.
2. Clearance The distance or dimensional difference between two parts. ( Remember to stack those tolerances up
)
2. Clearance The distance or dimensional difference between two parts. ( Remember to stack those tolerances up
jsharp nailed it.
Clearance........the distance between my oil filter, and my 400 degree header.
Tolerance......the time it takes for me to start cussing like a sailor when my hand exceeds the clearance.
Tolerance......the time it takes for me to start cussing like a sailor when my hand exceeds the clearance.
MolaKule
Staff member
Yep, the guy from Illinois (Mr jsparp) nailed it.
Another term for Clearance might be:
1. space between adjacent parts,
2. the distance by which one surface clears another.
SBC, you crack me up; been there, done that!
Another term for Clearance might be:
1. space between adjacent parts,
2. the distance by which one surface clears another.
SBC, you crack me up; been there, done that!
This just proves that cats are smarter than people..they get burned once and dont repeat the mistake. People know the header is hot and still try to get their hand in there.quote:
Originally posted by sbc350gearhead:
Clearance........the distance between my oil filter, and my 400 degree header.
Tolerance......the time it takes for me to start cussing like a sailor when my hand exceeds the clearance.
Says the one who burned his hands setting valve lash on a hot slant six today....
Dan
MolaKule
Staff member
BTW Dan, how are the "Doctoral" studies coming along?
There's another definition around here. Clearance is a town about 6 miles east of Paxton Illinois on Illinois Rt 9.quote:
Originally posted by MolaKule:
Yep, the guy from Illinois (Mr jsparp) nailed it.
Another term for Clearance might be:
1. space between adjacent parts,
2. the distance by which one surface clears another.
SBC, you crack me up; been there, done that!
Wait, that's Clarence...
Used in a sentence:
Jane says the piston to bore clearance is out of tolerance according to the motor manual.
Jane says the piston to bore clearance is out of tolerance according to the motor manual.
I used to work with a chef who was fond of saying, "Didja ever notice how people will pick up something that's too hot to touch and put it directly in their mouth?"
Very good guys......you're sharp!
But can anyone calculate the nominal clearance plus tolerance on the following 2 parts (this is a quiz but I'll give the correct
answer)
Shaft (be it a crankshaft journal or whatever) 2.250 +.000/-.001 Dia.
Bearing I.D. 2.251 +.001/-.000
Answer: .001 (nominal clearance) +.002/-.000
or put another way: a clearance of from
.001 to .003
So now you know as much as I do (but they pay me for stuff like that!)
LOL...................Lukey
But can anyone calculate the nominal clearance plus tolerance on the following 2 parts (this is a quiz but I'll give the correct
answer)
Shaft (be it a crankshaft journal or whatever) 2.250 +.000/-.001 Dia.
Bearing I.D. 2.251 +.001/-.000
Answer: .001 (nominal clearance) +.002/-.000
or put another way: a clearance of from
.001 to .003
So now you know as much as I do (but they pay me for stuff like that!)
LOL...................Lukey
That's an unusual definitioln of "nominal clearance". Your "nominal clearance" would never occur unless the manufacturing process were out of control and a lot of parts were being rejected. Not exactly a nominal condition.quote:
Originally posted by lukey:
Very good guys......you're sharp!
But can anyone calculate the nominal clearance plus tolerance on the following 2 parts (this is a quiz but I'll give the correct
answer)
Shaft (be it a crankshaft journal or whatever) 2.250 +.000/-.001 Dia.
Bearing I.D. 2.251 +.001/-.000
Answer: .001 (nominal clearance) +.002/-.000
or put another way: a clearance of from
.001 to .003
So now you know as much as I do (but they pay me for stuff like that!)
LOL...................Lukey
"Nominal clearance" more often means the mathematical mean total clearance.
When unilaterral dimensions are used as in your example, the fact that the tolerance is given all in one direction is only a way to indicate which direction is the safest to deviate from standard.
Nominal clearance in your case would more commonly and more correctly be given as 0.002 with a tolerance of +/- 0.001 on the clearance.
Whether you dimension a part with unilateral or bilateral toleranceing has no effect of the actual "nominal clearance".
MolaKule
Staff member
Tarzan says clearances must have iuncreased!
I disagree......there would be no parts rejected
if the clearance were .001. It's still in the accepted tolerance range.
It's all a word game......the point is.....in manufacturing these parts, one has to plan on
a clearance of from .001 - .003
Lukey
One can start creating a disconnect if design tolerances don't match manufacturing tolerances. The uniltaeral tolerincing is common with shafts and holes, but when making parts one typically sets up a bilateral tolerance, which would be 2.2495 +/_ 0.0005 and 2.2515 +/- 0.0005 using the above. The resulting clearance would be 0.002 on average with a tolerance range of +/-0.001, assuming centered normal distributions. Regardless of what the designer wants he really needs to bite the bulltet and to use realistic tolerances in order to produce a robust design. Manufacturing needs to center their process, minimize variation, and constantly work to improve the process.
Some case studies mentioned in tolerancing classes noted that awhile back Toyota had worked to improve their manufacturing tolerances on large assemblies, while Ford just made bigger holes in order to get parts to assemble; guess who had more rattles down the road :^)
Another one compared transmissions, trying to determine why Toyota transmissions lasted longer than Ford units even though both were assembled with parts in spec. Ford had ended up doing a fair amount of sorting to get rid of bad parts, endeding up a distrbution of parts truncated at the spec limits, while Toyota ended up with tighter distributions by keeping theirs centered with less variation. The result was Toyota had assembled more units with parts closer to nominal, so the assembled units perfromed closer to nominal. Wheeler described sorting out bad parts as 'scraping burnt toast'.
Some case studies mentioned in tolerancing classes noted that awhile back Toyota had worked to improve their manufacturing tolerances on large assemblies, while Ford just made bigger holes in order to get parts to assemble; guess who had more rattles down the road :^)
Another one compared transmissions, trying to determine why Toyota transmissions lasted longer than Ford units even though both were assembled with parts in spec. Ford had ended up doing a fair amount of sorting to get rid of bad parts, endeding up a distrbution of parts truncated at the spec limits, while Toyota ended up with tighter distributions by keeping theirs centered with less variation. The result was Toyota had assembled more units with parts closer to nominal, so the assembled units perfromed closer to nominal. Wheeler described sorting out bad parts as 'scraping burnt toast'.
Some of the japanese engine builders used to measure every part and mark them where they fell in tolerance. Usually ID'd as a A, B or C. Then upon assembly they could hand select for a very accurate clearance spec. Not sure if things are still done this way.
Kestas
Staff member
To add some confusion, the bearing industry sometimes refers to preload as 'negative clearance'.
Our aircraft structures drawings are usually dimensioned nominal in inches with a this tolerance statement in the title block,
DECIMALS:
.XX 2 PL [+-symbol] .03
.XXX 3 PL [+-symbol] .010
DECIMALS:
.XX 2 PL [+-symbol] .03
.XXX 3 PL [+-symbol] .010
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