Patent ID: 7854258

Claim:
A downhole tractor comprising: first and second grip assemblies each having a power stroke and a return stroke; wherein the first grip assembly comprises a plurality of first engagement members, each for contacting a well formation; wherein the second grip assembly comprises a plurality of second engagement member, each for contacting a well formation; wherein the plurality of first engagement members each remain in contact with the well formation during both the power stroke and the return stroke of the first grip assembly; and wherein the plurality of second engagement members each remain in contact with the well formation during both the power stroke and the return stroke of the second grip assembly, wherein the plurality of first engagement members each have a first coefficient of friction with the well formation during the power stroke of the first grip assembly and a second coefficient of friction with the well formation during the return stroke of the first grip assembly, and wherein the first coefficient of friction is higher than the second coefficient of friction.