Motor selection

Hi,

I'm looking for a type of motor that has high torque and inertia  to drive a spring load application. The mechanic of it would be very similar to the reciprocating saw, where the motor torque would be converted to vertical movement. Instead of cutting, I'll use it to pull a spring load (approximatly 20lbs for distance of 8 inches, to make a quick jig movement) Please advise on what type of motor would work best.

Thanks,

Trung