Vulcan Onshore Tip #68: Hammer Blow Rate

Very frequently users and engineers make the assumption that the output energy of a hammer is somehow related to the number of blows per minute the hammer puts out. Such a notion is mistaken for both Single-Acting and Differential-Acting hammers.

Vulcan specifications for blow rates are computed with the condition of “Normal Stroke, No Set.” This means first that the hammer must be getting a full stroke. This is the foremost method for Single-Acting hammers to determine sufficient energy and should be the main method to determine whether or not a hammer is performing properly.

With Differential-Acting hammers, the pressure at the hammer is the deciding factor. The second part of Vulcan blow rate specifications state that the blow rate is computed with no pile set considered. This includes both consideration of the time it takes to impact the pile and the effects of rebound on the hammer.

The interaction of the pile is an important component in determining the blow rate of a hammer and is extremely variable with driving conditions. Some hammers (such as the single-acting diesels) can have their energies estimated with their blow rates. Because of the action of the air supply, this cannot be done with Vulcan air-steam hammers.

Users who need alternative methods to determine the energy output of the hammer should measure the ram velocity directly or use other accepted methods.

Note: this and Tip 9 deal with a subject that recurs with Vulcan hammers, and has been a source of confusion with some geotechnical engineers.

When Vulcan first introduced the Super Vulcan Differential-Acting hammers, it included with them a chart which correlated the blow rate of the hammer with the output energy. Unfortunately, this chart did not take into consideration either the time of impact or the rebound energy from the pile. Since Vulcan hammers (except for the diesel) were not designed to require rebound to achieve full stroke, the presence of initial velocity after impact seriously affected the blow rate and compromised the usefulness of the charts. Vulcan withdrew the charts and spent many years trying to live down their blowback.

The success of the diesel hammers–and the use of the Saximeter to estimate their rated energy–led some to attempt such a correlation with Vulcan single-acting air/steam hammers. Unfortunately no correlation can be established because of the different nature of the upstroke of these hammers and again because of rebound effects. But the result–confusion–remained.

Although a excessively slow hammer may indicate problems with binding or inadequate lubrication, there are many variables that go into a hammer’s actual blow rate in the field, and thus the factory wisely opted out of any further correlations between blow rate and energy for any of its hammers.