04813498 is referenced by 12 patents and cites 10 patents.

An active counterbalance for suspending a power swivel from a traveling block. The power swivel includes a motor drive assembly for rotating a drill string and a handling system for supporting a drill pipe. The counterbalance includes a motor which provides means for biasing the load capacity of the counterbalance when supporting the power swivel to allow vertical displacement of the power swivel relative to the traveling block. The counterbalance prevents damage to connecting or sealing surfaces when adding pipe to or removing pipe from the drill string because most of the weight of the power swivel and pipe suspending from the handling system is transferred to the counterbalance.

Title
Active counterbalance for a power swivel during well drilling
Application Number
7/163580
Publication Number
4813498
Application Date
March 3, 1988
Publication Date
March 21, 1989
Inventor
David B Ford
Frisco
TX, US
James E Lynch
Dallas
TX, US
Agent
L A Fillnow
R H Johnson
R J Bunyard
Assignee
National Oilwell
TX, US
IPC
E21B 19/06
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