No, the Cut-In Height Didn’t Cause the “Downfall” of the Air Hammers

There’s an urban legend going around social media these days that states that the “demise” (which is hardly true since they’re still in use) of air hammers is due to the fact that the valve admits air to the cylinder before impact, thus slowing down the ram.  This legend has it that this was detected by the PDA, thus people switched to diesel hammers.

The distance between the turn of the valve and impact on the down stroke is referred to in Vulcan terminology as the “cut-in height” and generally runs anywhere from 1 1/2″ to 2 3/4″.  There are two reasons why this is part of the hammer’s cycle:

  1. The pressurisation of the cylinder is not instantaneous.  When the valve first admits pressurised air into the cylinder, the flow is choked, i.e., at the speed of sound, and is limited by that until the pressure in the cylinder is about 55-60% of the inlet pressure.  Thus it takes time for the air under the piston to pressurise, and when the ram is nearing impact there isn’t much time.  For example, let’s take a 3′ stroke hammer with standard WEAP 67% efficiency.  The impact velocity of this hammer is 11.38 ft/sec, and to pass a 2 3/4″ cut-in takes 20 milliseconds.
  2. Since the end user determines how much cushion material is put into the hammer, the actual cut-in depends upon the cushion stack+top plate height.  It’s possible to put so much cushion into the hammer that the hammer will not run.  To put in a little “forgiveness” is useful on the jobsite.

It’s also worth noting that the PDA does not measure the impact velocity of the ram, but the force-time and velocity-time history of the pile head.  There have been impact velocity measuring devices for air hammers since the 1970’s but the PDA isn’t one of them.

The problems with air hammer efficiency are in part due to one of the advantages of air hammers: because of their simplicity, they will operate with minimal maintenance.  That minimal maintenance, however, means that air hammers will run but not necessarily well, especially with long-term deficient lubrication and dirt in the system.  This takes its toll on the efficiency of the unit and end users should be aware of this when putting their hammers back into service.

With diesel hammers, because of the heat generated during combustion diesel hammers are intolerant to poor lubrication; they either run or they don’t, all other things equal.  This forces the end user to keep them lubricated, which helps in their efficiency.

Now we come to an important topic in air trapped under the ram just before impact.  When a diesel ram falls towards impact, the ports are closed and the air compresses between the ram and the anvil.  The energy for this compression comes from the falling ram and is necessary for the thermodynamic cycle of the hammer.  (See the diagram to the right.  With an air or hydraulic hammer, the combustion thermodynamics take place in the power pack or compressor, outside of the hammer.)  Nevertheless this still requires kinetic energy from the ram, and some agencies “derate” the rated single-acting energy of the diesel hammer by ~20%, which is a typical ratio of the compression energy to the rated energy of the hammer.

One thing is for sure: the WEAP family of wave equation programs was the first to incorporate a reasonable model of diesel hammers, and that definitely was an assist for the diesel hammers gaining broader acceptance.

Air hammers and diesel hammers have relative strengths and weaknesses, and this has been discussed before on this site in another context.  These should be considered by the end user when selecting a hammer for a particular project.  The most important reason for “downfall” with air hammers is gravity, but that’s part of the design too.

Photo at top: Vulcan hammer getting ready to drive pile on a project for the Port Authority of New York and New Jersey.  Photo courtesy of Pile Hammer Equipment.

 

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