1. Field of the Invention
This invention is directed to a rotary for use in a power tong.
2. Description of Related Art
In drilling operations it is necessary to connect and disconnect successive lengths of tubulars such as drill pipe which are provided with threaded connections. The tools used for holding the pipes while they are being screwed together are known as tongs. Power tongs generally include a rotary for rotating the pipe while a back-up tong holds the pipe.
Tongs are generally provided with one or more jaws which can be moved into engagement with a pipe. In one typical arrangement, two "passive" jaws are fixed in a tong and a third "active" jaw is advanced towards or retracted away from a pipe as desired. In one power tong the active jaw is mounted in a jaw holder, the radial extremity of which is provided with a roller which rests on a cam surface formed on a rotary. When the rotary rotates relative to the jaw holder the roller rides along the cam surface and urges the jaw against the pipe with a force which is a function of the slope of the cam surface. Once the jaw is firmly applied the pipe and rotary rotate in unison.
Such power tongs can only be used with pipes having a relatively small range of diameters. In order to accommodate substantially larger or smaller diameter pipes it is necessary to change the cam surface either completely or by adding wedges. This is a time-consuming and expensive procedure.
In order to overcome this problem it has been proposed to provide a rotary for use in a power tong, which rotary comprises a slave hydraulic piston and cylinder, a jaw carrier associated with the slave hydraulic piston and cylinder, and means to selectively retain hydraulic fluid in the slave hydraulic piston and cylinder.
In this arrangement, a master hydraulic piston and cylinder are mounted alongside the rotary. In use, the master hydraulic piston and cylinder are connected to the slave piston and cylinder by a hose having a releasable coupling. Hydraulic fluid is then pumped from the master hydraulic piston and cylinder to the slave piston and cylinder to apply the jaw(s). Once the desired pressure is reached, valves on either side of the coupling are closed and the releasable coupling is disconnected to enable the rotary to be rotated. In order to release the jaws the hose is reconnected, the valves are opened and the hydraulic fluid is allowed to return to a reservoir.
The repeated coupling and disconnection of the releasable coupling is a time-consuming process and great care has to be taken to ensure that the releasable couplings are correctly attached and that the hoses connected thereto are not damaged.
There has long been a need for a tong which does not require such time-consuming coupling. There has long been a need for a tong in which hydraulic fluid need not return to a reservoir. There has long been a need for such a tong which can efficiently and effectively handle pipe of varying diameters.