here is info i am using for many years for my robots off the servo city site
Servo power is stated as ounce-inches (oz-in.) This is the maximum amount of power the given servo will apply with a one inch arm. For example, if a servo has a power rating of 16 oz-in, the maximum amount of power it will be able to apply with a 1" arm will be 1 lbs.
If you are needing to find out how many pounds a servo will lift or push with a 1" arm, simply divide the oz-in. number by 16.
Servo speed is stated in seconds. Just as a car has a 0-60 mph time, a servo has a 0-60 degree time. The lower the 60 degree time the faster the servo operates.
oz-in. / 16 = pounds of force (1" arm)
If you are planning on using a 2" arm, you will then need to divide the pounds of force once again by 2.
An HS-425BB has 57 oz-in. of power
57 divided by 16 = 3.56
3.56 would be the pounds of force with a 1" arm
If you were to put on a 2" arm on the HS-425BB:
3.56 / 2 = 1.78
1.78 would be the pounds of force with a 2" arm
If you were to put on a 3" arm on the HS-425BB:
3.56 / 3 = 1.18
1.18 would be the pounds of force with a 3" arm
second is when you look at the servo for torque rating the stated oz-in is stall torgue not the torque you need its much less ,its about 75% of the stall torque rating
to get higher torque is a little hard,need a gear box ,lets say you have 100 oz servo and you need a 300oz out,ratio is 3-1 ,3 times the input torgue but the 3 times less the speed ,to make a gear box is easy looking at lets say 12 tooth gear on servo and
36 on the output gear,second you need a 5k pot on the output gear to keep it at the right speed,so you need to remove the the pot from the servo,better to use a high grade 5 k pot,then the cheap one used in servo's
also there is a simple test to check the lift on the servo very easy.most servo comes with arms ,i think the longest is 3 inches ,but may be more you need to go from the screw on the servo to then end of the arm and all has holes and put a weight on it,and have a amp meter inline with you power ,for servo stall (smoke) and add weight till gets close to it
that is put it will lift with that arm ,if you want add a 12 inch arm ,but MATH is never wrong divide 12 inches by the arm lets say 3 inch arm = 4 then ,take the weight it can lift and divide by 4,
16 oz is the max weight it can lift divide by 4 = 4 oz,its only as example depends on the servo under test,when wanted to lift a arm its the full length need to use ,plus the weight of the item you want to lift
hope this info is not to hard for some to understand,but at work i test motors for torque using very high cost machine ,plus designing and building gear boxes for many years,
so ask any question i can help