Wanting to build a lifter but don't know what kind of reduction you'll need to be able to lift your enemies? Sure, you could just throw in the first motor you find on eBay and hope it works, or you can use this handy dandy calculator below to do the work for you! All you'll need are the following parameters to get started:
LIFTER ARM LENGTH (ft)
If you don't know the amount in feet, you can do the following:
- in to ft - Divide by 12
- mm to ft - divide by 304.8
MAXIMUM LIFTING WEIGHT (lbs)
Check with your local competition to see if weight bonuses are allowed. It's always safe to assume that, if a competition does allow for weight bonusues, there will be at least one person who will attempt to create a robot at the maximum weight limit, so be prepared to create a lifter that can lift 2x its own weight.
MOTOR STALL TORQUE (oz-in)
Most motors, especially those found online, have stat sheets online that list the stall torque. If not, you can ask in one of the groups or websites mentioned in the "Resources" section. Or, you could do the following equation:
STALL TORQUE (oz-in) = (1352 x Voltage) / (Kv x Ri)
Kv = RPM per Volt.
Ri = Internal Resistance (Ohms).
Note that most motors online will have Internal Resistance listed somewhere. If you cannot find that, see the link below:
STALL TORQUE (oz-in) = (1352 x Voltage) / (Kv x Ri)
Kv = RPM per Volt.
Ri = Internal Resistance (Ohms).
Note that most motors online will have Internal Resistance listed somewhere. If you cannot find that, see the link below:
TORQUE OVERAGE FACTOR
This number represents how much speed the lifter will have under its maximum load, as well as how hard it will be pushed towards its maximum torque. With a torque overage factor of 1.67, the motor will be loaded with no more than 60% of its stall torque rating when lifting the maximum load, and will travel at at 40% of its listed speed. For the best results in terms of lifting speed and safety from damage via overload, do not set this number below 1.5 or above 2.
The power you'll need from a gearmotor (or hacked servo) will depend on the length of the lifter arm attached to it. For reasonable speed and reliability, select a gearmotor that will stall with no less than 1.67 times the weight class on the end of the lifter. The calculation to ensure your motor will suffice is as follows:
The power you'll need from a gearmotor (or hacked servo) will depend on the length of the lifter arm attached to it. For reasonable speed and reliability, select a gearmotor that will stall with no less than 1.67 times the weight class on the end of the lifter. The calculation to ensure your motor will suffice is as follows:
- Minimum Stall Torque (oz-in) = length of lifter arm (inches) * weight class (ounces) * 1.67
A FEW NOTES BEFORE YOU PROCEED:
- This program is designed to calculate the torque necessary for a single-pivot lifter system. For four-bar lifter systems, follow this link!
- Stall torque, stall current, and no-load RPM of a brushed PMDC motor increase proportionally with voltage. Increasing voltage from 18 volts to 24 volts, for example, will increase the motor’s stall torque by 1.5. Therefore, if the motor’s stall torque is 166 oz-in at 18 volts, it will be 249 oz-in at 24 volts.
Insert your variables below!
Lifter Arm Length (ft)
Maximum Lifting Weight (lbs)
Motor Stall Torque (oz-in)
Torque Overage Factor
Special Thanks to David Small and Mark Joerger!