Precision Alternative for Spring Tension users? The turnbuckle
 

Hey all,

I thought I'd make a thread for the development of this idea!
I sort of brainstormed this idea after doing a lot of front engine and WYIT stuff when doing the belts, and I have to say that even though the tensioner bolts A and B are tightened to correct torque spec and the locknut has been replaced and the car previously did many thousands of miles without a spring in the spring tensioner, I am a bit concerned that there is nothing back there as a safety/insurance reinforcement on the tensioner arm. (Previous owner removed the spring) So, I came up with the idea of designing a turnbuckle like the PS and alternator belt turnbuckles that would replace the spring on a spring tensioner in late model cars. The advantages - No need to use a balance shaft pin spanner combo tool to mess around with the tensioner arm to get correct tension, no weakening given that there is no spring to apply incorrect tension, and of course very fine and strain-free adjustment given that the adjustment would simply be a turn of the turnbuckle body. Any thoughts? I've designed it with a 14mm hex body, a pivot point in order to compensate for the slight changes in geometry when adjusting tension, an eye bolt with bushing to help insulate slightly against vibration (not sure if this is necessary, engineers/guys who know a lot please comment), set screws in the body to keep the turnbuckle together when not in the car, and a screw through the mounting pad (not yet implemented since I do not know the dimensions of the hole in the recession for the tensioner spring on the mounting bracket for the tensioner) in order to keep the turnbuckle anchored to the mounting bracket in case it would attempt to work itself out of the recession.

What I need, though, is someone to very graciously take some measurements with a pair of calipers for me if they have any spring tensioners lying around. I would very much like to machine this in the machine shop but I need very precise measurements so I don't make it too large/small etc. I did do a rough model based on an overhead image by scaling the image until the bolt head in the image was roughly 13mm and modeling off that, so it should be close. What I would need are the measurements for distance between mounting points (farthest and closest), diameter of that recession pocket for the spring in the tensioner bracket and the diameter of the hole that is in it, diameter of the pin on the tensioner arm that the spring wraps around, and length (technically depth) of that pin. If anyone is able to provide at least some of these, that would be incredibly awesome.

Thanks for taking a look!

GTR


http://forums.pelicanparts.com/uploa...1420688768.jpg

http://forums.pelicanparts.com/uploa...1420688788.jpg

I like the ingenuity, but if you are that concerned why not just switch to a still-sprung tensioner?

Well, like I mentioned, older tensioners have a reputation for not applying correct tension especially to new belts. So, if I wrestle with the pin wrench and then secure the nut and bolt, there might as well not even be a spring in there cause I didn't really use it to get to full tension. This would eliminate the pin spanner and doing two jobs at once (pulling on the tensioner arm while tightening the nuts). I feel like it would provide a much more precision adjustment when adjusting with the tension gauge.

I have had a couple of cars with these tensioners and did not observe a problem. I think you could make a pin wrench to 3/8 square adapter and make it pretty easy to hold the tension with one hand and tighten a bolt with the other. I guess I would also question if the spring is really not setting the tension adequately or are people trying to over-tension a new belt so it's close to OK when it wears in and "stretches." IMO it's better to retension after just a few dozen miles and then again after 1000 miles or so than it is to stress the system (water pump bearing) to compensate.

Sine you say "older tensioners have a reputation for not applying correct tension especially to new belts" and the tensioner does not know the age of the belt I assume you want it tighter with a new belt. This can be obviated with a couple quick retensiones early on in the belt's life and that will be friendlier to the other bits.

-Joel.

Interesting point, seems like a valid idea. I agree with less stress on the bearings also.

Perhaps you could adapt the 968-style hydraulic tensioner to your motor?
Or you could drill another hole in the swing-arm for a pin/bolt to lock everything into place (additional to the pivot bolt and the locknut)

The 968 system is nice but the retrofit is probably way too much work compared to a few re-tensions. If someone make a kit like the PorKensioner for the 928 that might change things. In a 951, setting tension is more of a pain but in the NA cars it's no big deal. I think it'd be risky to retrofit or bodge a tensioner on the car given the carnage if it fails.

I'm not sure what you mean about a locking pin or bolt, it already has a locking bolt and pivot nut. You'd need a magic pin that locks in in just the right place so you could tighten the fasteners at your leisure. The spring does this if it's not worn out and maybe if it is you shim it or replace it?

Hmm, shimmimg it, never thought about that! Easy to buy or machine a shim. Only thing is, does anyone know where to buy just the spring if it's possible?