Mark II Geometry Reduces Rod Load and Peak Torque
Lufkin Mark II Unitorque Pumping Units are available in fifty different sizes up to the M-1824D-427-216; a unit with a 216-inch stroke, a polished rod capacity of 42,700 pounds and a 1,824,000 inch-pound double reduction gear box.
The Mark II’s unique UNITORQUE geometry is characterized by three (3) basic configurations that reduce the rod load and peak torque over that of a conventional pumping unit. The design characteristics which make the Mark II unique are:
- Shifting the gearbox from directly under the equalizer towards the Samson post turning in a preferred direction of rotation, creates an upstroke that occurs in approximately 195° of crank rotation and a downstroke of approximately 165° of crank rotation.
- Placing the equalizer between the horsehead and the Samson post creates a “push-up” or “Class 3” lever system.
- The 195° upstroke, coupled with the front mounted geometry, reduces the acceleration at the beginning of the upstroke where the load is greatest, thereby effecting a reduction in polished rod load. Locating the cross-yoke forward of the gear reducer creates a greater mechanical advantage for lifting the heavy load on the upstroke, and a lesser mechanical advantage for the reduced downstroke load (i.e., the maximum upstroke torque factor is reduced while the maximum downstroke torque factor is increased).
- An angular offset in the crank that produces a more effective counterbalance torque which at the beginning of the upstroke “lags” the well load torque approximately 71⁄2 degrees. Similarly, at the beginning of the downstroke, this same offset condition produces a counter balance torque which “leads” the well load torque approximately 71⁄2 degrees.
Independently these features would not produce a relatively uniform torque, but working together a “unitorque” system is obtained which can reduce the torque on the gearbox up to 35% as well as lowering power costs and often, prime mover size.