Interchangeability of manufactured parts is a critical element of present day production. The production of closely mating parts, although theoretically possible, is economically unfeasible. For this reason, the engineer, designer or drafter specifies an allowable deviation (tolerance) between decimal limits.
The definition of a Tolerance, per ASME Y14.5.5M-1994, is the total amount a specific dimension is permitted to vary. For instance, a dimension shown as 1.498” to 1.502” means that it may be 1.498” or 1.502” or anywhere between these dimensions. Since greater accuracy costs money, you would not callout the tightest possible tolerance, but instead would callout as generous a tolerance as possible.
Definition of Terms
Example 1
Maximum Material Condition (MMC) – Is the condition where a feature of a finished part contains the maximum amount of material. That is, the largest shaft or smallest hole. See Example 1.
Least Material Condition (LMC) - Is the condition where a feature of a finished part contains the least amount of material. That is, the smallest shaft or the largest hole. See Example 1.
Nominal Size – Approximate size used for the purpose of identification such as stock material.
Basic Size – Is the theoretical exact size from which limits of size are determined by the application of allowances and tolerances.
Tolerance – The total amount by which a given dimension may vary or the difference between the limits.
Limits – The extreme maximum and minimum sizes specified by a toleranced dimension.
Allowance – An allowance is the intentional difference between the maximum material limits (minimum clearance or maximum interference) of mating parts.
Refer to Example 1 above: MMC of the hole – MMC of the shaft = Allowance.
MMC Hole = 1.250
- MMC Shaft = 1.248
Allowance = .002
Fits
Clearance fit – A clearance fit results in limits of size that assure clearance between assembled mating parts.
Refer to Example 1 above: LMC of the hole – LMC of the shaft = Clearance.
LMC Hole = 1.251
- LMC Shaft = 1.247
Clearance = .004
Interference fit (also referred to as Force fit or Shrink fit)– interference fit has limits of size that always result in interference between mating parts. For example, a hole and shaft, the shaft will always be larger than the hole, to give an interference of metal that will result in either a force or press fit. The effect would be an almost permanent assembly for two assembled parts.
Example 2
Least amount of Interference is:
LMC Shaft = 1.2513
- LMC Hole = 1.2506
Min Interference = .0007
Greatest amount of Interference:
MMC Shaft = 1.2519
- MMC Hole = 1.2500
Max Interference = .0019
Transition fit – A transition fit might be either a clearance or interference fit. That is, a shaft may be either larger or smaller than the hole in a mating part.
Example 3
LMC Hole = 1.2506
- LMC Shaft = 1.2503
Positive Clearance = .0003
MMC Shaft = 1.2509
- MMC Hole = 1.2500
Negative Allowance (Interference) = .0003
Basic Hole System – The basic hole system is used to apply tolerances to holes and shafts assemblies. The minimum hole is assigned the basic diameter (basic size) from which the tolerance and allowance are applied. This system is widely used in industry due to standard reamers being used to produce holes, and standard size plugs used to check hole sizes accurately.
Computed Clearance Fit using Basic Hole System
.500 = hole basic size .500 basic hole
.002 = Allowance (decided) - .002 allowance
.498 Maximum shaft
Step 1 Step 2
If tolerance of part is = .003 then:
.498 maximum shaft .500 basic hole
-. 003 tolerance +.003 tolerance
.495 minimum shaft .503 maximum hole
Step 3 Step 4
Calculate clearances:
.500 smallest hole (MMC) .503 largest hole (LMC)
-. 498 largest shaft (MMC) -.495 smallest shaft (LMC)
.002 minimum clearance .008 maximum clearance
Step 5 Step 6
Drawing annotation of tolerance
Example 3
Basic Shaft System – The basic shaft system can be used for shafts that are produced in standard sizes. When applying this system, the largest shaft is assigned the basic size diameter from which the allowance for the mating part is assigned. Then, tolerances are applied on both sides and away from the assigned allowance. One situation for using the basic shaft system is when a purchased motor, with an attached shaft, from which a mating hole must be calculated.
Computed Interference fit using Basic Shaft System
.500 = shaft basic size .500 basic shaft
.002 = Allowance (decided) - .002 allowance
.498 Maximum hole
Step 1 Step 2
If tolerance of part is = .003 then:
.498 maximum hole .500 basic shaft
-. 003 tolerance +.003 tolerance
.495 minimum hole .503 maximum shaft
Step 3 Step 4
Calculate clearances:
.498 largest hole (LMC) .495 smallest hole (MMC)
-. 500 smallest shaft (LMC) -.503 largest shaft (MMC)
- .002 minimum interference -.008 maximum interference
Step 5 Step 6
Drawing annotation of tolerance
Example 4
Preferred precision fits – The American National Standards Institute publishes the “Preferred Limits and Fits for Cylindrical Parts” (ANSI B4.1-1967) to define terms and recommending standard allowances, tolerances, and fits for mating parts. The chart data is provided in thousandths (.001) of an inch. For example: -1.2 and -2.2 (See Example 5) for calculation purposes would be -.0012 and -.0022.
Running and Sliding fits (RC1-RC9)
Loosest of the class fits, used when a shaft is must move freely inside a hole or bearing, and the positioning of the shaft is not critical. This fit would always allow a clearance between shaft and hole.
Clearance locational fits (LC1-LC11)
Tighter than RC fits, but the shaft and hole may be the same size. LC fits allow the shaft to be located more accurately than the RC fits but may still be loose. With this fit, a shaft would move less freely inside a hole.
Transition locational fits (LT1-LT6)
These fits are a compromise between LC and LN (interference/force) fits. These fits would allow either a small amount of clearance or interference.
Interference locational fits (LN1-LN3)
Used where accuracy of location is the prime importance such as alignment of dowel pins and other devices where location relative to another part is of prime importance.
Force and shrinks fits (FN1-FN5)
With this fit, the shaft is always considered larger than the hole. These fits are used to transmit torque such as a motor shaft to a bearing.
Limits Calculations Using ANSI B4.1 Standard Tables
Class RC6 Clearance Fit
Partial Table from ANSI B4.1
Example 5
A nominal hole size of .8750 Diameter and a RC6 Class Fit has been selected.
Hole nominal size range = .71 – 1.19
Minimum clearance = .0016
Maximum clearance = .0048
Tolerance of hole = +.0020, -.0000
Tolerance of shaft = -.0016, -.0028
Calculations:
Hole: Basic size .8750 .8750
Tolerance +.0020 -.0000
Maximum hole .8770 Minimum hole .8750
Shaft: Basic size .8750 .8750
Tolerance -.0016 -.0028
Maximum shaft .8734 Minimum shaft .8722
Limits of size for Hole and Shaft
Example 5
Limit Calculations when one Design Feature Exists
When calculating the limit tolerances for features that mate with purchased parts, the purchased part size must be known. This may be obtained be requesting a drawing from a vendor or, a caliper or micrometer can be used to obtain an accurate size.
Example:
A shaft diameter of .2500 is to be pressed into a part using a FN4 interference (force) fit.
Limits of size for the shaft diameter are .2500 and .2495.
The table shows a minimum acceptable interference of .0006 and maximum interference of .0016.
Calculations:
Maximum shaft: . 2500
Maximum interference: -. 0016
Minimum hole: . 2484
Minimum shaft: . 2495
Minimum interference: -. 0006
Maximum hole: .2489
References:
Dimensioning and Tolerancing, ASME Y 14.5M-1994, The American Society of Mechanical Engineers.
Technical Drawing Tenth Edition, Frederick E. Giesecke, Prentice Hall, Upper Saddle River, NJ 07458.
Geometric Dimensioning and Tolerancing, 2003, David A. Matson, Goodheart-Wilcox Co. Inc., Tinley Park, Illinois.
Prepared by Bill Bussard\
http://www.maelabs.ucsd.edu/mae_guides/Tolerance/Tolerancing.htm
http://www.tec-ease.com/gdt-tips.php
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