The size of machine suitable for compression tests on 2" × 2" sticks or for 2" × 2" beams with 26 to 36-inch span has a capacity of 30,000 pounds.
SPEED OF TESTING MACHINE
In instructions for making static tests the rate of application of the stress, i.e., the speed of the machine, is given because the strength of wood varies with the speed at which the fibres are strained. The speed of the crosshead of the testing machine is practically never constant, due to mechanical defects of the apparatus and variations in the speed of the motor, but so long as it does not exceed 25 per cent the results will not be appreciably affected. In fact, a change in speed of 50 per cent will not cause the strength of the wood to vary more than 2 per cent.[58]
Following are the formulæ used in determining the speed of the movable head of the machine in inches per minute (n):
| (1) | For endwise compression | n | = | Z l |
| Z l2 | ||||
| (2) | For beams (centre loading) | n | = | ------ |
| 6h | ||||
| Z l2 | ||||
| (3) | For beams (third-point loading) | n | = | ------ |
| 5.4 h | ||||
| Z | = | rate of fibre strain per inch of fibre length. | ||
| l | = | span of beam or length of compression specimen. | ||
| h | = | height of beam. |
| The values commonly used for Z are as follows: | |||
| Bending large beams | Z | = | 0.0007 |
| Bending small beams | Z | = | 0.0015 |
| Endwise compression-large specimens | Z | = | 0.0015 |
| Endwise compression-small specimens | Z | = | 0.003 |
| Right-angled compression-large specimens | Z | = | 0.007 |
| Right-angled compression-small specimens | Z | = | 0.015 |
| Shearing parallel to the grain | Z | = | 0.015 |
Example: At what speed should the crosshead move to give the required rate of fibre strain in testing a small beam 2" × 2" × 30". (Span = 28".) Substituting these values in equation (2) above:
| (0.0015 × 282) | ||||
| n | = | ----------------- | = | 0.1 inch per minute. |
| (6 × 2) |