Calculating/Choosing Spring tension and Springs to assist servo/actuator movement

I'm currently working on an animatronic that requires the raising and lowering of a small scissor platform.  The most efficient way I figure to do that is to use a linear servo/actuator to push the lower support bar.  the total travel for this is 4 inches, though the lower it gets the more force is required.  After mulling it over and chatting with a friend, spring assists seem to be the way to go for this.   My question is, how does one calculation proper spring tension and choose the right springs for the job.   Should I keep an assorted spring set on hand for future projects and if so are there general recommendations for sets (McMaster Carr?) 


  • You should be able to get a spring assortment from McMaster Carr or Amazon.  I just got mine off Amazon, and it's good to get both compression and expansion springs.

    As far as the calculations, that I don't know.  A proper engineer would be able to comment there.  The ratings for your servo can usually be found on the manufacturer's site or possibly in the paperwork that came with the servo.  It's usually rated by how many ounces it can lift if the payload is 1 inch out from the point of rotation.  So a rating of 44.4 oz-in means on a 1-inch arm with the weight at the end, it will lift 44.4 ounces and still keep its proper speed.   Anything over that load will start to slow it down and stress it, which can result in stripped gears or burnt-out components.

    If you can calculate the weight required to push your scissor mechanism with various springs attached you could determine if it's within an acceptable range for your servo, given the attachment point for your servo and its mechanical advantage.
    Push/pull force gauges can help there. 


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