- Clearances and Tolerances -
Misused Terms and Misunderstood Concepts
The misuse of the terms "clearance" and "tolerance" in contemporary technical writing appears to be an increasingly frequent occurrance, which interferes with the clear communication of technical information. (For further evidence of our deficient and degenerating education system, see BELOW.)
"Writers" who should know better often treat these two terms as if they refer to the same concept, and as if they are therefore interchangeable. More than a few of these writers blather on with self-bestowed authority, while perpetuating manifest technical and grammatical errors. This short page is an attempt to clarify, within the context of mechanisms and manufacturing, the meaning of the two terms "clearance" and "tolerance" and the obvious differences between them.
The Merriam-Webster® dictionary contains four different definitions for the word "tolerance". The definition ( non-politically-correct ) which is applicable to this context is: "the allowable deviation from a standard; especially the range of variation permitted in maintaining a specified dimension in machining a component."
That same dictionary contains three basic definitions (and multiple examples) of the word "clearance". The definition which applies to this context is: "the distance by which one object clears another, or the clear space between them."
Stated simply, CLEARANCE is the distance between the adjacent surfaces of mating parts (how much the mating parts CLEAR each other), while TOLERANCE is the allowable variation of a dimension from its nominal (desired) value ( the amount of error one will TOLERATE ).
As an example, let's discuss the fit between the main journals of a crankshaft and the engine bearings which support that crankshaft (and allow it to rotate with relatively low friction). Suppose we have a crankshaft in which all the main bearing journal outside diameters (OD) are exactly the nominal (desired, ideal) diameter of 2.4488 inches. Further, suppose that the inside diameter (ID) of all the main bearing bores in the engine block arel exactly the nominal of 2.6411 and that all the main bearing shells are exactly the nominal thickness of 0.0953.
Basic arithmetic reveals that the ID of the main bearings (as installed in the block) will be 2.4505.
2.6411 - (2 x 0.0953) = 2.4505
Further basic arithmetic reveals that the crankshaft journal OD's are 0.0017 smaller than the main bearing ID's.
2.4505 - 2.4488 = 0.0017
That difference (0.0017) is the bearing CLEARANCE, (the distance by which the journal outside surface CLEARS the bearing inside surface). That clearance, as explained in the engine bearings page, has a dramatic effect on the load carrying and frictional properties of the journal-bearing system. That 0.0017 nominal value, BTW, is a good number for this size bearings in a production engine for a road vahicle.
In real life, however, it is a rare case when the dimensions of a manufactured component are all exactly nominal. Contemporary manufacturing processes enable parts to be made with ever smaller variations from nominal at an acceptable cost; however, variations still do occur. Those variations from the nominal dimension are the TOLERANCES applied to the part ( the variation from nominal that will be TOLERATED ).
Going back to the example, the production TOLERANCE on the 2.4488 crankshaft journal OD is +/- 0.0005 (plus-or-minus 0.0005 inches), which means that any particular journal can have a diameter anywhere from 2.4483 to 2.4493 (a variation of 0.001 total). Similarly, the manufacturing tolerance on the main bearing bores in the block (in this example) is +/- 0.0005, which means that the ID of any main bearing bore can vary from 2.6406 to 2.6416. If we ignore the tolerance in the bearing shell thickness (assume that all bearing shells have the nominal 0.0953 thickness), then the journal and bore tolerances mean that the production clearance between any main journal OD and its mating main bearing ID can vary from as little as 0.0007 (2.6406 bore and 2.4493 journal) to as much as 0.0027 inches ( a 2.6416 bore and a 2.4483 journal).
Naturally, in a precision high-performance engine, the bearing clearances are held to a much smaller tolerance (+/- 0.0001), but have a (typically) larger clearance (0.0026). Those very small tolerances, coupled with appropriate clearances and exact axial alignments, can produce an engine in which the crankshaft, with all 5 main bearings torqued to spec, and with the front and rear oil seals installed, can be spun with the flick of the wrist and would make you think the crank was running in rolling element bearings instead of journal bearings.
The subject matter of this page is directed toward one specific error that occurs all too frequently in contemporary "technical writing". The fact that there are SO MANY OTHERS is a depressing statement about our degenerating educational system..
For example, it is commonplace today to read published articles in which the author apparently does not even know the difference between THERE, THEIR and THEY'RE.
Or between BARE and BEAR; or between BRAKE and BREAK; or COURSE and COARSE; or CHORD and CORD; or CAPITAL and CAPITOL; or DUAL and DUEL; or FAIR and FARE; or PLANE and PLAIN; or PRINCIPLE and PRINCIPAL; or PAIL and PALE; or RAIN, REIN and REIGN; or ROLL and ROLE; or SHEAR and SHEER; or STEEL and STEAL; or VERSUS and VERSUS; or VICE and VISE....... the list goes on.
It is apparently an elusive concept that the spelling of a word can dramatically alter the meaning, and a different word that SOUNDS the same does not convey the same meaning. That often leaves the reader to try and figure out what the writer actually means.
There are apparently many other elusive language concepts such as the identity, structure and use of a subject and a verb in a sentence; the agreement between nouns, verbs and adjectives with regard to number and gender, and other pre-high school language basics. ( And let's not even BEGIN to discuss the abyssmal comprehension of fundamental arithmetic being conveyed in our schools.)
We also see the increasingly-common usage of "dominate" in place of "dominant", which then extends to the erroneous use of "predominate" in place of "predominant", as in ".....iron is the predominate element in this steel alloy "...(instead of the correct... predominant element... ) .
Which then brings us to the phenominally ignorant practice of using "apostrophe-s" to create a plural form (as in "....this assembly requires eighteen bolt's...").
While I am ranting, I might as well add the all-too-common use of the term "oil galley" to describe a drilled passage (usually in an engine block) which conducts oil from one place to another.
A GALLEY is the kitchen on a ship; an oil GALLERY is the oil passage.
STIFFNESS vs. STRENGTH
A common example of totally-technically-wrong writing occurs in articles on improvements to engine parts, in which the author (or promoter) claims that the use of a higher grade of steel alloy in a particular product has added stiffness to the product, when in fact, if the "improved" part has the same external dimensions as the original part, the stiffness isIDENTICAL to the original part. The yield and tensile strength of the component MIGHT be higher, depending on the heat treatment of the parts. The basic difference between strength and stiffness is explained further HERE.
In closing, itt appears to me that one can become a "technical writer" with only a smattering of technical knowledge and a 4th-grade comprehension of the English language. Some "authors" are even too sloppy to use spellcheck.