Idler tensioning system

[Adrian Harris]

One of my worries about the Sherman track tensioning system is that the whole track tension is held by a shearing force on the M3 screw in the idler bracket. The thought of trying to extract a sheared screw from this bracket gives me the heebie jeebies.

I know there is absolutely no evidence that this system has ever given anyone any trouble but I thought I would see if I could come up with a more prototypical system

The real Sherman uses a splined section of the idler shaft, onto which slides a similarly splined plate. The plate is held in place with a spring clip on the top of the idler bracket and the whole shaft is prevented from rotating by the plate interfacing with a machined step on the back plate of the idler bracket.

BARV Idler Tensioner.jpg (74.34 KiB) Viewed 778 times

As the Armortek idler shaft has a square section machined onto it, I have had a set of plates cut with a matching square hole, with the hole rotated at various angles to facilitate the tensioning of the track. The plate butts up to the back of the idler bracket to prevent the idler shaft from rotating, as per the original design.

Model Idler Tensioner.jpg (43.98 KiB) Viewed 778 times

To hold the plate in place, I have machined a blind M8 nut and washer, from hex and round brass stock respectively, to replicate the part of the idler shaft which extends beyond the locking plate.

This is still a work in progress, as I'm currently working on a modified plate and I'm also planning affixing a section of metal to the back of the idler bracket, to replicate the step to which the plate abuts.

I also need to replicate the small spring clip which holds the plate in place and add the cotter pin which stops the 1:1 plate from sliding off the idler shaft altogether.

Adrian.

[Allan Richards]

Thats a nice piece of work. You will find that the Sherman needs its tracks quite tight to run well but this said, if anything gets caught in the tracks the suspension will compress to releive the tension.

[Dan Williams]

I do wish the sherman model had a better system,Because once I removed a link,and had a hard time getting the track tight.The screw was in the third hole,and one link pin failed.The track came apart,the track link pin look like the letter "S".But I need to try this again,To remove a link and get the track very tight.
I like what your working on.Are you able to look at the real parts taken from a rebuilt Sherman ?
Dan

[Adrian Harris]

The Sherman does run best with a tight track.

I used to have an M32 recovery Sherman just a couple of minutes drive away but it has now been moved for possible restoration, so I'm working from photographs mainly.

Adrian.

[Fabrice Le Roux]

Hi Adrian,

I know this is something I mentioned when we chatted the other day, but others may not be aware.

The full size sherman tracks were "live" links not "dead" links like our models. This means that a rubber bush isolated the pin from the track pad and could be twisted when fitted to impart a torsional spring effect to each link.

In practice this meant that the track wanted to curl upwards at the disconnected ends. When connected, the links wanted to curl round the idler and sprocket, reducing the tendancy to sag behind the sprocket and get crimped, while reducing rolling resistance.

A less obvious effect was the tendency to resist sag between the steel springs atop the bogie units. Indeed any marked sag was a sign of track pin bush failure according to the vehicle manuals.

So the conundrum is that while the tracks run best when tight, the scale model tracks are not live, so try to sag under gravity. To tighten the model tracks to make them look prototypical there is a risk of putting considerable load on the tensioning mechanism, which could lead to failure of the retaining bolt or of the track itself.

Personally I have elected to run the track a little less tight and use a mild steel capscrew to lock the plate, so it might fail rather than the track.

For more adventurous model engineers, a more refined solution might be to use an adjustable torsion spring concealed within the housing to provide adaptive track tensioning.

cheers
Fabrice

[Dan Williams]

With running in grass,the right track started to come off all of the wheels.
I rev the track to get them back on.And some times the bogie wheels come together before I can stop it.
But running on hard surface I have no problems.
Dan