Bolt Travel

TIMEtSEC.)

Figure 1-28. Time-Travel and Time-Velocity Curves for First 0.010 Second

Evaluating this expression for the conditions of the example gives:

26 V21 92

The bolt driving spring must be proportioned so that it will absorb this amount of energy over the entire distance through which the bolt moves in recoil. The energy absorbed by a spring is equal to the distance through which it is compressed times the average force required to produce this deflection.

That is:

Er=FavD or Fftv=~

If it is assumed that the bolt in the example must open 10 inches (0.833 feet) in order to permit feeding of a 20-mm cartridge, the average force exerted by the spring must be:

In designing the spring so that it will produce this average force, the same factors described in the analysis of plain blowback should be considered. However, an arbitrary choice of spring characteristics will suffice for present purposes. If the initial compression is taken as 130 pounds, a maximum force of 336 pounds will produce the required average force of 233 pounds. Since the diffcrcncc between the maximum force and the initial comprcs-

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