Rubber Tolerances Vs Metal Tolerances

As explained in a previous article, it is significantly easier and cheaper to achieve tight tolerances for metal parts than it is to achieve the same tight tolerances for rubber parts. Table 1 shows typical tolerances for extruded rubber parts and Table 2 shows typical tolerances for machined metal parts.

Table 1: Extruded rubber tolerances – ISO 3302.

Nominal dimensions (mm) E1, ± (mm) E2, ± (mm) E3, ± (mm)
Above Up to and including
0 1.5 0.15 0.25 0.40
1.5 2.5 0.20 0.36 0.50
2.5 4.0 0.25 0.40 0.70
4.0 6.3 0.36 0.50 0.80
6.3 10 0.40 0.70 1.00
10 16 0.50 0.80 1.30
16 25 0.70 1.00 1.60
25 40 0.80 1.30 2.00
40 63 1.00 1.60 2.50
63 100 1.30 2.00 3.20
100 - 1.30% 2.00% 3.20%

Table 2: Linear dimension tolerances for machined parts – ISO 2768.

Nominal dimensions (mm) Tolerance class designation
Above Up to and including f (fine), ± (mm) m (medium), ± (mm) c (course), ± (mm) v (very course), ± (mm)
0.5 3 0.05 0.10 0.20 -
3 6 0.05 0.10 0.30 0.50
6 30 0.10 0.20 0.50 1.00
30 120 0.15 0.30 0.80 1.50
120 400 0.20 0.50 1.20 2.50
400 1000 0.30 0.80 2.00 4.00
1000 2000 0.50 1.20 3.00 6.00
2000 4000 - 2.00 4.00 8.00

The four main reasons why rubber tolerances are so loose are:

  1. Rubber is a soft material –it is difficult to hold tighter tolerances on soft materials because the measuring method often causes deformation, which can significantly alter the measured value.
  2. Rubber is natural –natural rubber is an organic material that comes from live trees; therefore, there can be material variances from batch to batch that will create relative tolerance variations.
  3. Rubber manufacturing process –variations, such as rubber shrinkage or swelling, can occur during manufacturing, sometimes affecting the tolerances because they cannot always be predicted.
  4. Rubber is compounded –other ingredients are added to the natural or synthetic rubber to help with the curing process, this mixture of added ingredients can affect the amount of swelling or shrinking.
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