This post could be about the effects of c, velocity ripple and smooth motion, the differences between linear drive technologies or any number of topics. However, one thing is more important than everything else. Life.
Big surprise, life is important. No - not the Milton Bradley board game and No - not "What happens to you while you're making other plans." We're talking about the life of your linear motion products. The bearing industry has adopted a standard of B10 (or L10) ratings, but the standards aren't so clear cut in the linear positioning world. If we look at the life of linear positioners then we need to determine life ratings for the actuator as a whole, which would include the linear bearings, the screw, the axial bearings, and any flexing or bending component.
Comparing linear actuators
So why are we so compelled to write about this topic? Simple, most manufacturers adhere to their own life criteria for specifying product payload capacities. This means that comparing two linear positioners may not be as easy as looking at the load capacity of one versus another. Manufacturers rate actuator load capacities at different points on the load/life curve, some manufacturers rate their payload capacities for their actuators based on 100km of travel (reference their CKR catalog).
Rating for a short distance results in extremely high payload capacities and it may appear that one product is inferior to another when in actuality it may be superior. Other manufacturers (e.g. Parker Origa) rate their products for 8,000 km resulting in more nominal load capacities. So the manufacturers really determine their normal load or thrust capacity based on where on the life curve they want to be. The question becomes: Do you need the positioning system to carry the world for a day or half of the world for a year?
Critical factors to determine life of unit
To create apples to apples comparison, you really need to make sure you are looking at the life of the unit with the given application details. Ultimately this will be affected by:
- effective payload
- the speed of motion
- duty cycle of various payloads
- environmental contamination
- the sizes of the components selected
It is important to calculate the life for the weakest link, for example a rodded electric cylinder might have a very light payload to move (and seemingly a very long life) but may be encountering a pressing application and significant forces that would limit the ballscrew life. The alternate might be an HD actuator moving a very heavy payload but at a very slow speed with almost no axial force. The weakest link in one application might be the thrust bearings or the screw (see ballscrew life standards ISO-3408-5) and in the other it might be the linear square rail bearings.
Importance of knowing expected life
Ultimately it's important to know what type of life you can expect with the positioning system you are looking at. It could cost you more in the long run if you don't. Now if I could just get the industry to adopt my idea of standardizing on actuator classes (based on drive grade, flatness, sealing, accuracy, etc.) similar to ballscrew classes, it would be much easier to weed out the weak.
Download our whitepaper to learn more about guidelines in designing a machine for IP69K environment.
Ben Furnish - Market Development Manager Automation Group Parker Hannifin
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