## DG & DG2 series: service life calculation example

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### DG & DG2 series: service life calculation example

Q: My customer is considering your DG2 series hollow rotary actuators because they like the large hollow bore, and they're asking what type of life can be expected from this unit (model DG200R-ARAAD2-3). They are planning on using this as a positioning drive similar to a rotary arm indexer. It will rotate a 35kg arm/gripper assembly 180deg about 10,000 times per week. I'm running no more than 50 RPM on the DG2 table. I estimated about 25 Nm of required torque with safety factor. Can you help me with an estimate on the life expectancy for this application?

A: We can definitely help you determine the estimated life of the product for your customer's specific application. We can do this one of three ways, and let me explain each method in detail.

1. Use reference life of 10,000 hours for gear life. HOWEVER, this is the most conservative and least accurate method since we're assuming the product is operating under max conditions: max torque, speed, thrust load, overhung load, motor current, ambient temperature....etc. Motors should not be sized for its maximum capabilities. Therefore, actual life would most likely be longer, and it really depends on your specific application conditions and parameters. There are many ways to increase life such as using a motor at half the current and keeping operating temperature lower.

2. If you already have the DG2 series hollow rotary actuator, you can measure the motor case temperature after the temperature has been saturated and stabilized during operation. You can contact us with the measured temperature, and we can estimate life based on temperature.

3. The most accurate way to measure life expectancy is definitely to perform an actual test of our motor and driver with your specific load, speed, operating parameters and environmental conditions. If you cannot perform this test, then we can try to estimate life by calculation with the variables given. However, calculation is only as accurate as the variables used.

Here's the formula:

L (Life in hours) = L1 * ( K1 / (K2^3 *F))

http://www.orientalmotor.com/products/p ... h_life.pdf

L1 (rated gear life); see page G-36 of the document above, table 1.

K1 (speed coefficient) = reference input speed / actual input speed

K2 (load factor) = operating torque / permissible torque

-unidirectional continuous operation.
-belt conveyors and film rollers that are subject to minimal load fluctuation.

-frequent start/stops.
-Cam drive and inertial body positioning via stepper motor.

-Frequent instantaneous bi-directional operation, start/stops of reversible motors.
-Frequent instantaneous stopping by brake pack for AC motors.
-Frequent instantaneous start/stops of brushless motors and servo motors.

L1 = 10,000 hours because DG gear rated life is 10,000 hours (same as GFS type gearheads).

For K1, we need to reference the input speed for the gearhead (motor speed) from the application. For the reference input speed, I used 3000 RPM (same as GFS type gearheads). I used 900 RPM as the actual input speed because you require 50 RPM at the DG2 table which has an internal gear ratio of 18:1 so 50 x 18 = 900 RPM.

For K2, the required torque is 25 Nm, and the permissible torque spec from the DG200 (50 Nm).

Use 2 for the load-type factor since this will be a more aggressive duty cycle. (10000 time per week)

See full calculations as explained below:

L = L1 * ( K1 / (K2^3 *F))

L = 10,000 * [(3000 RPM / 900 RPM) / ((25 Nm / 50 Nm)^3 x 2)]

L = 133,333 hours

Hours in a year = 24 x 7 x 52 = 8736

133,333 hours / 8736 hours per year = 15.26 years

In contrast, if we only used the 10,000 hour reference gear life to estimate the same life expectancy, then 10,000 hours / 8736 hours in a year = 1.14 years life. This further demonstrate the importance of calculation/testing vs using conservative reference values.

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