The Back Hammer is available in three sizes:
Type 140 - for 3" to 5" DTH hammers
Type 160 - for 5" to 6" DTH hammers, and
Type 190 - for 8" to 12" DTH hammers
The Back Hammer is a percussion mechanism in which the impact power is directed backwards. When needed it is installed in the drill string above the drill hole and is started by opening the throttle for the percussion air to the DTH hammer.
The Back Hammer is designed to help to free a DTH hammer and drill pipes that have jammed in the drill hole. It can also be used to hammer down or withdraw casings.
The Back Hammer is mounted between the rotating unit and the drill pipes. It consists of only five main parts: a protective barrel 6, which houses the piston 4, support ring 5, spindle 1 and plug 2. The spindle has a 3½" API Reg pin thread at the top and a 3½" API Reg box thread at the bottom. If the rotation unit or drill pipes have another thread size or type, the appropriate adaptors will be needed. Since the Back Hammer is mounted between the rotation unit and the drill pipes, rotational torque is still transmitted to the entire drill string when the Back Hammer is fitted.
When the air throttle for the percussion mechanism of the DTH hammer is opened, the piston in the Back Hammer starts to strike the support ring 5.
This must be combined with light reverse feed force and rotation to the right. It is important always to have rotation to the right switched on; otherwise the joints in the drill string will loosen.
The path of the working air through the Back Hammer - and thus the impact and return strokes of the piston - is controlled by means of directional ducts in the piston 4 and spindle 1. On the impact stroke, the piston strikes the support ring 5, which transmits the impact energy into the drill string. The return stroke of the piston is braked by an air cushion. By means of a choke plug 2 fitted in the spindle 1, air can also be directed to the DTH hammer during Back Hammering.
The exhaust air from the Back Hammer is expelled through the bottom of the protective barrel 6.
For hammering down casings, the Back Hammer is simply mounted upside down, whereupon the impact energy is directed downwards.