ok so here is a little routine that will deffinatly give you a strong grip.
Grip training: mon,wed,fri 3 days a week
Warm up, No. 150 gripper 4 X 12 reps
No. 200 gripper 4 X 8 and 7 reps
No. 250 gripper closed as far as possible and held for as long as possible, 2 sets
Drop Set--No. 250 closed and held, 1 set
No. 200 closed 3x5 times and held
No. 150 closed 4x6 times and held
Hold 2 25 for as long as possible
3x max hold
over time if a gripper becomes easy, switch it.