View Full Version : OT Can someone explain how this works

02-23-2011, 02:45 PM
I installed a sensor bar on a printer the other day.
It's basicly a sensor that shines on the paper but can be moved across the page by turning a shaft.
The normal way they to do this is like a leadscrew the block with the sensor has a nut that runs on threaded shaft.
This one has a smooth shaft with 6 bearings 3 mounted on each side of the block but at a angle.
I can see how it works but can someone explain the "math" behind it.

02-23-2011, 03:05 PM
Lets see..
If the bearings were parallel to the shaft, they would just spin, and not move the carrage.
If the bearings were 90deg to the shaft, they would not spin, just lock up, and not move the carrage either.
If the bearings were 45deg to the shaft, half the rotation of the shaft would spin the bearings, and half the rotation of the shaft would move the carrage.
Sounds like a tangent function to me.
Just my 2 minute thought.


Lew Hartswick
02-23-2011, 05:38 PM
The HPLC (high pressure liquid chromatgraph) that I worked on at
PSU used that drive for the chart pen drive. Worked great and had
as close to ZERO backlash as could be measured. :-)

02-23-2011, 08:49 PM
The bearings 'walk' round the axle like the thread of a nut 'walks' around the external thread of a leadscrew. For a given shaft diameter, the longer the pitch, the steeper the helix angle. The mount angle of your bearings is the helix angle.

Take a long thin right-angled triangle, with the small angle being the angle of the bearing, the helix angle. Wrap it around the shaft so the hypotenuse is the helix, and the other long side is a circumference of the shaft. Cut down the triangle so it goes round the shaft exactly once. You have one thread, and the small side of the triangle is the pitch.

Err... This is meant to be a hypothetical exercise, for illustration.