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Ponoko RepRap - Installing the Z Axis Tensioner

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The Z Axis Tensioner is a device that clamps to the RepRap frame and puts pressure on the Z drive chain to keep it taught. This is necessary as the chain can only be joined at intervals of one link and as the Z drive is in use it wears in slightly. A floppy chain will vibrate and skip out of the drive gears, thus we use the Tensioner.

The amount of tension applied can be varied by moving the Tensioner on the frame, changing the lengths of soft plastic tubing used as spacers, and by tightening the nuts that compress the tubing.

The tubing spacers also act as shock absorbers, dampening vibrations in the chain and absorbing the bumps caused as balls pass over the Tensioner's Idler.


Here are the parts needed to build the Z Axis Tensioner. Actual thicknesses of these parts are not critical and tend to vary from kit to kit depending on where the free cutting space is. Later kits also have notches in the two brackets (clear parts, top right) to accommodate washers.

DO NOT cut the tubing yet. The tubing serves as a combined spacer and shock-absorber. If you have a lot of slop in your Z chain you will need longer pieces of tubing. If everything is nice & tight, you may need less or no Tensioner at all! Measure it up first.

Parts include:

  • 5 off M3 25mm machine screw
  • 4 off M3 12mm machine screw
  • 10 off M3 nuts
  • 14 off M3 washers
  • 2 off M5 50mm machine screw
  • 2 off M5 nut
  • 4 off M5 washers
  • 2 off <part p="2">5mm i/d soft plastic tubing, approx 22mm long</part>

<repeat count="2">


Put <part p="2 M3">washer</part>s on <part>two M3 12mm screw</part>s and put them into the circular plate. The holes are symmetrically placed. Put an <part p="two">M3 nut</part> on the end of each of the screws.


Attach the first Z Tensioner Bracket to the round plate using the by now familiar captive nut principle. Do not fully tighten - you will need to manoeuvre the Brackets to accommodate the idler. </repeat>


Attach the second bracket, and prepare a <part>M3 25mm screw</part> for use as the axle of the Idler by slipping a <part p="M3">washer</part> onto it. Adjust the gap between the Brackets so that the 8mm-thick Idler will be able to rotate freely and then tighten the four screws holding the Brackets.


Slip the M3 25mm screw through the two Brackets and the Idler. Follow it up with a <part p="M3">washer</part> and lock <part>two M3 nut</part>s off after it (use an M3 nylock nut instead if the fancy takes you). Ensure that there is plenty of slack and the Idler rotates freely.


Put <part p="2">M5 washer</part>s on the <part>two M5 screw</part>s, and slot them into the arcing slots on the round plate. Take a look at the last photograph and hold the assembly in place. You should now be sure which slots the M5 screws go in, and you can measure the length of tubing needed - add a little extra as you will be compressing it. Cut two pieces and slide them onto the M5 screws.


Fix the diamond-shaped plate to the M5 screws with <part p="2 M5">washer</part>s and <part p="2 M5">nut</part>s. Put the <part p="4 M3">washer</part>s on the remaining <part>four M3 25mm screw</part>s and push them through the rectangular plate. Now you are ready to secure the Tensioner to the RepRap frame with the remaining <part p="4 M3">nut</part>s and <part p="4 M3">washer</part>s.


When finished, the Tensioner looks like this. Do not over-tension the chain! If the Tensioner stops the chain skipping and vibrating madly on your Z axis it is tight enough. Over-tensioning will increase chain and Z axis wear leading to far too much slop, and then you'll have to cut a link out of the chain.

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-- Main.RussNelson - 30 Dec 2008 -- Main.VikOlliver - 30 Dec 2008