How the Timing Belt Tensioner Actually Moves

[Maddog]

So I have always been curious how much the timing belt tensioner moves while going through the rpms. I built an engine test stand for a different project and decided I would try to measure how much the tensioner moves. I always assumed that the tensioner plunger moved inward as the rpms increased. I thought this was because the belt would get forced outward more and more as rpms climbed. I wouldn't say I ever analyzed the system though.

Anyway I took a video today of a dial indicator that I placed against the lever arm for the tensioner pulley. Its hard to get in that area in general and I couldnt get an exact measurement relative to the plunger. However you could assume the movement is more than the dial indicator shows. The video is boring and unedited but the first part of it was with the engine starting at 1000 rpms. Every ~10 seconds I increased the rpms by 1000 up to 5000.



I was surprised to see that the tensioner plunger actually moves outward as the rpms increase. I thought maybe the magnetic base of the dial indicator was moving around but when I let it go back to idle, the plunger actually slowly moves back inward. In the second part of the video I increase rpms to 5500 and slowly bring it back to idle. Again you can see the tensioner does the same thing. Its very repeatable which makes me think the measurements are fairly accurate.

I don't know exactly why the system is working this way but my assumption is the belt is stretching slightly and the placement of the crankshaft pulley and tensioner pulley prevent the belt from moving outwardly in that area. From the tensioner pulley to the first cam gear I can see the belt expand outward as you would normally expect. From idle to 5500 rpms the dial indicator shows movements of about 40 thousandths. The plunger itself will actually move more than that.

Thoughts about related subjects:
I wonder if a kevlar belt makes the tensioner act the same way? They claim they dont stretch... Anyone want to send me one to try this out on?
If you are using a solid tensioner, are you actually loosing some performance as the cam timing would be off a little because it can't expand outward?
I was always worried the washer mod for the tensioner would potentially cause the tensioner pulley bolt to loosen up because the plunger couldn't move inward and the belt would be trying to push it that way. It appears this isn't the case.

Thanks
Dan

[RealMcCoy]

Not surprising at all... When you put a greater load on the belt, it pulls all the slack out of the belt in the loaded areas, (and attempting to stretch the belt) which creates slightly more slack on the non-loaded section of the belt. The auto tensioner moves out to take up that slack.

I would expect the same behavior from a kevlar belt, although it may not be to the same degree being the belt teeth are stiffer and won't flex as much.

The change in tensioner length you are seeing will not equate to a measurable change in cam timing. If you do the math on a 20 inch length of belt, pushing the middle of it .100" off center, you've only changed the length of the belt span by about .001"

Regardless of which way it moved, the forces on the tensioner are the same.... I don't understand the fear of the pulley coming loose? Unless you somehow overtightened the belt, and had it bottomed out, there will always be the same pressure on the tensioner.

[CoopKill]

I agree it is just taking up slack from the front cam back to the crank, where all the slack ends up at the tensioner side (unloaded). Cool vid for sure!

[Primus101]

Great vid- at idle, you can see that the plunger is depressing as it gets hotter. Do you think you got enough temp in her to see the full expansion of the heads & block?

[Maddog]

Not surprising at all... When you put a greater load on the belt, it pulls all the slack out of the belt in the loaded areas, (and attempting to stretch the belt) which creates slightly more slack on the non-loaded section of the belt. The auto tensioner moves out to take up that slack.

I would expect the same behavior from a kevlar belt, although it may not be to the same degree being the belt teeth are stiffer and won't flex as much.

The change in tensioner length you are seeing will not equate to a measurable change in cam timing. If you do the math on a 20 inch length of belt, pushing the middle of it .100" off center, you've only changed the length of the belt span by about .001"

Regardless of which way it moved, the forces on the tensioner are the same.... I don't understand the fear of the pulley coming loose? Unless you somehow overtightened the belt, and had it bottomed out, there will always be the same pressure on the tensioner.

I am surprised that the belt stretches that much that the tensioner needs to take up that slack. I assumed the centrifugal force was greater than the spring force inside the tensioner. I had never seen anyone post that the plunger would actually move outward as rpms increase. I'm not an ME and I didn't do any real analysis. I just knew it allowed movement in both directions. After watching the video its easy to assume whats happening.

I don't know how the belts are made but I assume that the length of the belt has very little to do with the teeth flexing. I would assume they might be inner woven but there is some cording or something that controls the stretch of the belt independent of the teeth.

My fear was that yes by doing the washer mod (welding or putting washers at the end of the tensioner to prevent a gap between the lever arm and tensioner body) , it would bottom out the lever arm and the belt would put a force on the tensioner idler pulley that rotates it counterclockwise causing the bolt to loosen up.

Primus the coolant temps started at 190F and went up to 213F. I have an old radiator hooked up and a box fan blowing some air across it so it can only do some much when im revving the engine that high. I wanted to make sure the block, belt, and components were up to temp before doing any measurements. I have a video of the movement from a cold start to warm idle.

What types of scenarios would cause the plunger to go further inside the tensioner? Changing from high rpms to low rpms like shifting gears would put a force on it but the orifices inside of the plunger should prevent much movement. I'm still trying to see if the washer mod has a downfall. Better yet you could just set tension so that the lever arm is almost contacting the tensioner body. If the tensioner fails, it should keep enough tension that the belt wouldn't slip. Just thinking out loud.

[familyMAN]

Awesome vid. Thanks for posting. Always had concerns with kevlar and fixed tension. Guess you could run a kevlar belt with stock hydraulic and see if you get similar movement. If so, I would assume running fixed tension would be less than ideal, even with kevlar belt. If it doesn't move, or very little, then kevlar and fixed may be a good combo if you fear stock component failure.

[sergechronos]

Awesome vid. Thanks for posting. Always had concerns with kevlar and fixed tension. Guess you could run a kevlar belt with stock hydraulic and see if you get similar movement. If so, I would assume running fixed tension would be less than ideal, even with kevlar belt. If it doesn't move, or very little, then kevlar and fixed may be a good combo if you fear stock component failure.

Given unknown lengths of time that the OEM tensioners have spent sitting on shelves, I absolutely do fear stock component failure at this point.

Surely someone has a used kevlar belt that they can send to get this tested. Would love to see the results. Also Maddog, didn't see a video on your youtube of the cold start to warm idle, have you posted it yet or do you just have it? Did you have the dial indicator there for that as well? One of the biggest arguments heard against the solid tensioner and kevlar belt is that the hydraulic is supposed to adjust for the expansion of the engine components as it heats up, but your video starting at warm idle shows a pretty small spacing there anyhow, so not sure how much movement that there is. Throw that video together with one with a kevlar belt (push comes to shove I'll send you mine when I order it from Chris just to get a definitive answer) to show stretch during running, would pretty much answer most questions and concerns folks have about it.

[green-lantern]

Great info Dan, I think I spy bonsai in the background


You going to make NG?

[Chris@Rvengeperformance]

Given unknown lengths of time that the OEM tensioners have spent sitting on shelves, I absolutely do fear stock component failure at this point.

Surely someone has a used kevlar belt that they can send to get this tested. Would love to see the results. Also Maddog, didn't see a video on your youtube of the cold start to warm idle, have you posted it yet or do you just have it? Did you have the dial indicator there for that as well? One of the biggest arguments heard against the solid tensioner and kevlar belt is that the hydraulic is supposed to adjust for the expansion of the engine components as it heats up, but your video starting at warm idle shows a pretty small spacing there anyhow, so not sure how much movement that there is. Throw that video together with one with a kevlar belt (push comes to shove I'll send you mine when I order it from Chris just to get a definitive answer) to show stretch during running, would pretty much answer most questions and concerns folks have about it.

the tensioners are stamped on top with the date. I ordered mine from IPS and even that way it was manufactured at the end of 2012. If you are worried about a tensioner sitting on a shelf in an oiled bag for 1 year then I think you'll worry yourself to death. Probably the ones from cherry hill are even newer, they are definitely still making these parts.

I have to imagine one of the functions of the hydraulic tensioners is to dampen harmonics that the belt may experience, which are caused by the valvetrain. Even just a spring would be better than the solid tensioners IMHO.

[RealMcCoy]

the tensioners are stamped on top with the date. I ordered mine from IPS and even that way it was manufactured at the end of 2012. If you are worried about a tensioner sitting on a shelf in an oiled bag for 1 year then I think you'll worry yourself to death. Probably the ones from cherry hill are even newer, they are definitely still making these parts.

I have to imagine one of the functions of the hydraulic tensioners is to dampen harmonics that the belt may experience, which are caused by the valvetrain. Even just a spring would be better than the solid tensioners IMHO.

Not only are they still making them, they still USE them in Hyundai and Kia.