Track slack

[Kipsley]

Some tanks have the drive sprocket on the front and some have it on the back. I've noticed a lot of tank models like to have the tracks kind of draped over the upper guide wheels, and so was wondering...... were they really like that?

I seem to think that if this tank began to move forwards it would be looking at losing a track. I'm I right? Because as soon as that drive sprocket begins to turn it will pull all that slack out and the track would be very loose in front. So should tracks be tight? Not my model btw.

 

[Andy H]

Well...it all depends on the tank in question. American Shermans had 'tight' tracks, for instance. Japanese Chi-Ha's however...

 

 

 

 

 

 

[Crayons]

Ideally, you want no more 100mm of up lift in tensioned tracks from the centre point. Under tensioned will result in the possibility of shedding or snapping a track (usually during a turn or at high speed). Over tensioned will result in excessive drive wear and strain as well as the high likelihood of breaking the track pins.

[pigsty]

A lot of "dead" tracks run a little slack.  If you watch film of them moving, many have a length of quite droopy track below the frontmost wheel (even if it's the idler).  When parked that may translate into sag between the return rollers, as above, or the front bit may still hang down.  Tracks have to be set up this way because, with suspension travel, you can't guarantee that all the wheels will always be in their neutral position.  Another contributing factor is track wear - they really do get longer over time as the connecting pins deform.  Too much slack will cause problems, which is why idlers have tensioners and part of track maintenance is to check them from time to time and extend them to take up excessive slack.  Eventually the tensioner reaches the end of its range and the track has to be changed, or a link or two removed.

 

"Live" tracks - ones with bushes in the connectors that pull them in on themselves - generally don't have as much slack in them, but it can still be seen, and of course some vehicles can't help having slack.  For instance, any M110 would still have had some slack behind the sprocket even after being fitted with Diehl tracks.

[RAF4EVER]

11 hours ago, pigsty said:

A lot of "dead" tracks run a little slack.  If you watch film of them moving, many have a length of quite droopy track below the frontmost wheel (even if it's the idler).  When parked that may translate into sag between the return rollers, as above, or the front bit may still hang down.  Tracks have to be set up this way because, with suspension travel, you can't guarantee that all the wheels will always be in their neutral position.  Another contributing factor is track wear - they really do get longer over time as the connecting pins deform.  Too much slack will cause problems, which is why idlers have tensioners and part of track maintenance is to check them from time to time and extend them to take up excessive slack.  Eventually the tensioner reaches the end of its range and the track has to be changed, or a link or two removed.

 

"Live" tracks - ones with bushes in the connectors that pull them in on themselves - generally don't have as much slack in them, but it can still be seen, and of course some vehicles can't help having slack.  For instance, any M110 would still have had some slack behind the sprocket even after being fitted with Diehl tracks.

What is a Diehl track?, in laymans terms. NEWBIE

[Dads203]

4 minutes ago, RAF4EVER said:

What is a Diehl track?, in laymans terms. NEWBIE

German made track in essence. 

[RAF4EVER]

2 minutes ago, Dads203 said:

German made track in essence. 

Model or Actual?

[Dads203]

1 minute ago, RAF4EVER said:

Model or Actual?

Actual, it’s named after the German company who manufactures the track, they did some for the M113’s, M109’s etc. 

[RAF4EVER]

oke. thank you

[Kingsman]

Tracks are heavy.  Usually over a tonne per side on a medium tank.  Tiger tracks were among the heaviest at about 5 tonnes each with links of about 30kg each.  Supporting that weight is hard: they will sag without support, and even with it.  Tensioning that weight is also difficult and requires strong sprockets and idlers and axles as the tracks are effectively trying to rip them off.

 

Tracks, as others have said, fall into 2 types, live and dead.  I'm ignoring modern all-rubber tracks here.  Most tracks are and have been dead.  The pins are loose in the links making them free to rotate, allowing the track to flex either way.  These will sag if not supported.  German, British and Russian WW2 tracks were all dead. 

 

A minority, like those on the M3/M5 Light Tanks (Stuarts) and M3/M4 Medium Tanks (Lees, Grants and Shermans and brethren) were live.  Here the pins are fixed into the links and cannot rotate so that the track will only flex one way and will not lie flat off the vehicle, but will tend to curl up.  These will not sag unless the pins have come loose, when the link is technically considered to be broken.  Checking for broken links is a daily task on tanks with these tracks, as is tightening the wedges that generally secure the links together via the end connectors.  But as it didn't really affect operation I suspect they would not be routinely changed in action.  Even with live track, the M3/M4 needed support rollers.  They began life with 3 rollers and ended with 3 higher rollers plus 3 skids.

 

All that being said, too-loose tracks are a short way to shed one so there would be an allowable amount of slack and sag.  Track tensioning would - or at least should - be a crew daily task.

 

There was and is a belief that having the drive sprockets at the rear was preferable because that directly pulled the tank over the track laid on the ground, whereas having it at the front pushed the track onto the ground and pulled it indirectly via the idler - perhaps wasteful of power.  Rear drive has become the norm for tanks although front drive is still normal for APCs etc.  Although that has as much to do with the location of the drivetrain and the proven undesirability of driveshafts through the vehicle.

 

The key is to look at the tank type you're modelling and see how the tracks on that behaved.

[Longbow]

Some have a lot more slack than others 👍

 

[Kipsley]

This all does make sense. I had no idea that a track could weigh so much, and those true life pictures have a lot of slack in those tracks. I used to operate a 25 tonne excavator, and the mechanics were forever making sure the tracks were tight. The one time I did lose a track was due to a fluid leak in a tensioner. It just refused to stay tight.

 

Anyhow, I have learned da great deal, so my day is not lost. Tank tracks do sag.... and quite a lot apparently. 👍

[Longbow]

Your thread made me think of this photo. 
A JS-2 in Friedland, 1945, with a completely missing idler wheel. 👍