Episode #25: The Effect of Moment Loading on a Solution’s Lifespan

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The Motion Control Show

Transcription from Corey’s conversation with Bruce in Episode 25: 

COREY:  In the last episode, Kent Martins showed us just how expensive a mistake can be if he is not given all of the load information, like the center of gravity.  So, I've asked another one of our great Application Engineers, Bruce Ng, to expand on this idea and look at the actual lifespan of the actuators because back in Episode 8 I taught you how the load ratings are produced by looking at the bearing life curves.  So, Bruce is going to show us simply how the lifespan of an actuator changes by simply moving the center of gravity of the load.  We're teaching you how to do this yourselves, but ultimately what our team does is to help you fill in all the gaps of the what-you-don't-knows.  I'm Corey Foster of Valin Corporation.  Let's see what we can learn from Bruce. 

BRUCE:  Sure.  So, there's a few things that determine the life of a product.  Ultimately, it's going to be the first component that fails, and usually that ends up being one of the components that's, you know, wearing and moving during normal use.  So, you know, cables are pretty common, but the main one we’re really concerned about are the bearings.  So, if we look at a calculator and pull a rodless actuator like this one, usually we see something like a carriage that has four trucks, one on each corner of the carriage, and then those trucks will be on something potentially like a square rail.  So, if we think about like a car as an example, you know, there's four tires that are supporting the load of that car.  And if you have the load centered between those four 4 tires, they're going to wear evenly.  When you start turning, or if your load is centered off to the side, then you're changing how much load each of the tires is bearing, so the ones that have taken more load are going to wear faster than the ones that are bearing less of a load.  So, the same happens inside one of these actuators.  

So, if we plug in some numbers to this calculator, we can look with a 25-pound load here.  We’ll use a 30-inch overall stroke.  And then we're going to add 2 segments for the motion profile.  One is going to be a 24-inch extension in 1 second, and then there's going to be a 24-inch retract in 1 second as well.  So, this calculator will spit out a few different actuators that could potentially work here.  The purpose of this example, we're going to look at this one here for, you know, hey, we need a motor that sticks straight up to save space or something like that, so we're going to include that in our compare list.  And the calculator will spit out a life calculation here.  So, this one is estimating something over 12,000 miles worth of life here.  And that just happens to be what this calculator maxes out at.  Now if we go back and resize this with the load, instead of being centered on the carriage where all four trucks inside are sharing the load evenly, and we push that 12 inches to the side, like so, and rerun that calculation, and we've already seen, uh, just like that, we're already down from something over 12,000 miles down to just over 5,000 miles.  And while that might seem like a lot, depending on your duty cycle and how long you’re looking for that to last, you know that could be the difference between 5 years and 12 years. 

COREY:  Are you saying that we could lose seven years of lifespan out of an actuator if we don't take that into account?

BRUCE:  Yeah, absolutely!  So, when you're looking at the life of a product or anything that effects the load and how it loads the bearings, we want to know that, so we can give more accurate estimates here. 

COREY:  Do you ever feel like you're harassing customers, asking stupid questions, when you're harassing them about the center of gravity? 

BRUCE:  Yeah, I think a lot of them don't necessarily realize how big of an impact that can potentially be, so it might feel like we're nitpicking or something like that, but again, if we need it to last 20 years, then we really want to know everything that's going on and how that's affecting the bearings inside that actuator. 

COREY:  For sure!  Good information.  Thanks, Bruce.  Appreciate it.

BRUCE:  No problem!  

COREY:  If you need any help in filling in all the gaps of the what-you-don't-knows, reach out to us at Valin.com.  We're happy to help.  And don't forget to check out the rest of my show at TheMotionControlShow.com or follow the #MotionControlShow.                                                                                                                                                                                                                                 
If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.