Determination of a specific tolerance is highly dependent on the specific spring design, material used, equipment used to manufacture the spring, and heat treating methods used in spring processing. The tolerances presented are "normally achievable." However, the spring manufacture should be consulted before finalizing. In several areas, different grade levels are available. One should be aware that selection of the highest grade (most stringent) could have significant cost implications.
Today, use of statistical controls allows both spring manufacturer and customer a high level of confidence that spring " properly toleranced" will meet or exceed the performance requirements. Once again it is important to have both the designer and the manufacturer work closely together to establish the "process capability" which will determine the quality level obtainable.
Actual quality levels (Cpk) on many tolerance dimensions can only be determined through process capacity studies.
Note: It is possible to obtain tighter tolerance with intervention of special equipment. These special process frequently add significant cost.
Compression Spring Load Tolerances:
Regression formulas derived from industry standards for helical round wire compression springs (based on springs with parallel sides and constant pitch.)
There are two factors to calculate before computing the load tolerance. The first factor, (Af), is based on wire (d) and meaning diameter (D). Since specification was formulated to work with metric input, you must convert English to data to metric.
Therefore for conversion to inches:
d = d * 25.4
D = D * 25.4
Also, the math operators are written to accommodate programming. The following symbols are represented in the formula.
* = multiplication
/ = division
^ = exponentiation (raised to the power of the number to it's immediate right)
c (index) = D/d
Af = 65.92 * (d ^ 3.3 / D ^ 1.6)
*(-0.84 * (c / 10) ^ 3 + 3.781 * (c / 10) ^ 2 -4.244 (c / 10) + 2.274)
The second factor (Kf) is based on active material (Na).
Kf = 1 / (3 * Na ^ 2 ) + 8 / (5 * Na) + .803
The tolerance formula is then calculated as shown below.
(If English, convert the load in pounds, to Newtons with p = p * 4.44822)
Tol = ± Af * Kf + (1.5 * p) / 100
Compression Spring Free Length Tolerances:
The following formula may be used to calculate the Free length tolerance for an unloaded compression spring:
FL Tol = ± (Af * Kf * Q / R )
Using our example:
FL Tol = ± (12.95864 * 1.11836 * 1) / 8.76
FL Tol = ± 1.654 mm (1.654 / 25.4 = ± . 065")
For unground springs, multiply FL Tol by 1.7
Extension Spring Load Tolerances:
Regression formulas derived from industry standard for helical round wire Extension Springs.
The Af factor for calculating extension spring load tolerances is identical to that of the compression formula.
Af = 65.92 * (d ^ 3.3 / D ^ 1.6)
*( -0.84 * (c / 10) ^ 3 + 3.781 * (c / 10) ^ 2-4.244 (c / 10) ^ 2-4.244 (c / 10) + 2.274)
The second factor (Kf), however, as shown below:
Kf = 5.61 /Na + .7
The tolerance formula is also identical to the compression calculation
Tol = ± Af * Kf + (1.5 * p) /100
Torsion Spring Load Tolerances (Torque):
Regression formulas derived from industry Standard for helical round wire Torsion springs.
The factor (Kf) = 54
The tolerance formula is then as shown below.
Tol = ± ( ( 1.3 * Kf * d ^ 3) / Na ^ .24 * c ^ .5) ) * Q
Torsion Springs for Doors:
Weight of the door x the center of gravity (in inches) = inch pounds needed to lift the door.
A tolerance of +/- 10% is the standard. Anything smaller than this must take into account the factors that influence the spring rate (the spring diameter, feed, and wire size variation). Generally speaking, the feed and wire size vary negligibly. Diameter variation is what primarily controls the variation on the spring rate.
The following formulas should be used to calculate the commercial rate tolerance and rate CPC, if necessary.
Source Reference; SMI (Spring Manufactures Institute) Technology Committee; 1949,1970, 2000; Testing and Tolerancing, Guidelines for Spring Testing Tolerancing; SMI Encyclopedia of Spring Design