I was thinking a lot about if a track tensioner is needed. It adds quite a lot to the complexity of the undercarriage, and one of the goals was to keep it simple. But after some testing, I found that I think it's needed. With the large weight and powerful motor, something has to give when for example rocks comes between the drive sprocket and the track, and it keeps the track tensioned so that it doesn't detach so easily.
I ended up with this:
In this picture:
- A milled idler mount in 12mm aluminum
- An idler sprocket with a ball bearing, 4SR16 (16teeth, 08B)
- Two springs
- Two very long M6 bolts where I cut off the heads
- 16mm steel tube, 18mm long, inner diameter 12mm
- 12mm steel tube, 18mm long, inner diameter 8mm
- 8mm steel tube, 18mm long, inner diameter 6mm
- M6 40mm bolt
- M6 spacer 4mm long
The track I'm using is an industry standard roller chain, but it doesn't have much room for an idler wheel. That's why I'm using an idler sprocket. It helps to avoid derailing the track. The chain type is British Standard 08B roller chain, similar to the ANSI #40 roller chain (1/2" pitch).
The springs are quite strong to give enough tension to the tracks. Currently they push 15kg - 18kg outwards on each track, depending on their position in the milled slot. This seems to be fine. It can't be to low, as the track would loosen when driving into something.
Ebay links:
- Idler sprocket:
http://www.ebay.com/itm/British-Stan...-/191502712638
- Springs:
http://www.ebay.com/itm/191650948305
- Steel tubes:
http://www.ebay.com/itm/111668876683
Temporarily holding back the tensioner with a strip while securing it with a bolt:
