Due to the short rapid blows of the piston in a down hole hammer, the need for high amounts of weight on the bit has been eliminated. A down hole hammer needs only sufficient weight to keep the bit tight on the bottom.

The Champion and Challenger series hammers are often said to have different operating characteristics than other competitive hammers. This is primarily due to the following:

1. Heavy piston weight
2. Longer and less frequent strokes
3. Higher kinetic energy developed.

Due to these characteristics, total weight on bit may have to be increased to eliminate drill string bounce. Drill string bounce can be noticed at the surface when drilling shallow holes. However, on deep holes the total weight on bit may need to be calculated to find the correct amount of weight.

Remember that the hydraulic pressure indicated on the gauge is not the actual weight on the bit in pounds (kgs). Actual weight on bit will vary from rig to rig depending on the diameter of the hydraulic cylinders and the weight of the drill string. It may be necessary on deep holes to use hold back to maintain proper weight on bit. This depends upon the weight of the drill string.

A good rule of thumb for a starting weight on bit is 500 lbs. per inch of bit diameter
(9 kg per mm bit diameter.)

Example:
	360 6-1/2" bit	500 X 6.5 = 3,250 lbs.
	360 165 mm bit	9 X 165 = 1,485 kg

Remember this is only a rule of thumb for a starting point, drilling conditions can vary on every hole. Adding more weight or feed pressure does not increase the penetration rate. Drilling with excessive weight on bit can only decrease bit life and increase the torque loading on the drill string.

Although drilling with too much weight is detrimental, drilling with insuffient weight on bit is equally as detrimental. The common terms used for insufficient weight on bit are:

Drilling under these conditions can generate several operational problems such as:

1. Carbide Pop Out - When the piston strikes the bit without the bit being tight against the bottom of the hole, inertia can force the buttons to pop out of the sockets.
2. Bit Shank Failure - If the bit is not held firmly against the bottom of the hole excessive longitudinal movement between the bit splines and the chuck splines can generate extreme temperatures. This can cause a transfer of metal between the bit and chuck commonly known as cold welding. Once heat damage has occurred failure is inevitable.
3. Residual Stress Failures - If the hammer is not held firm against the formation, the energy generated by the piston can not be transmitted correctly and a large portion will remain in the piston. These energy vibrations can initiate failures in many different ways including piston, feed tube failures or top head drive failures.