# JVN-Design Calc Question

When using the JVN calculator for linear motion, should you include the diameter of your sprocket(s) as the pulley diameter to get a better estimate of speed and amps? Example: Put 5.4" in the pulley for an elevator with #35 15t at the bottom to a #35 44t at the top. Or, is there a better choice/method on the JVN to estimate this? Thanks for any help!

Use the pitch diameter of the sprocket you’re driving with your motor as your pulley diameter.

If you’re driving the 15t sprocket with your motor, use 1.79"
If you’re driving the 44t sprocket with your motor, use 5.257"

Note: Current estimates in JVN’s calculator don’t seem to take efficiency of the gearing into account. So for elevators that experience significant amounts of friction or binding, your current draw may be above what JVN estimates.

You would use the pitch diameter of the driving sprocket attached to your gearbox. I assume the 44T sprocket is just an idler at the top of your system? The PD of a #35 15T sprocket is 1.790", so that is what you input for your drum size. Those numbers can be found in this document.

I am pretty sure the current estimates in that calculator are calculating “holding” current for the load you specify. It is correct not to take inefficiency into account for this calculation; all that inefficiency is helping to hold the load. A 20% efficient transmission or a 100% efficient transmission will require the same amount of motor current to keep the same load stalled (all else being equal).

Friction is pretty great, except for when it isn’t. A lot like gravity.

Depending on the kind of inefficiency, the holding current could be rather less, even. However, I still think there should be some indication as to the current draw considering efficiency as well. There was/is a thread on here about designing a lift with a worm gear as a reduction stage. And running the lift for several seconds to get to height. When you’re talking about that level of inefficiency, the drastic increase in current draw to move the system would seriously affect the performance. Possibly enough to prevent the system from working.

Thanks to all the responders! It’s awesome being. A rookie team in a competition like FIRST! Hope to see some of you at competitions this season.

The direction of the static friction (helping or hurting) depends entirely on how your control approaches (and behaves at) the static position.

You have to build the smarts into the controller to “back off” and let the static friction help hold the load.