T-belt Tension Tool

DR sent me the Official Timing Belt Tensioner and I have been playing :-)
The object of my games is to make a cheap alternative. Here are my results, please anyone feel free to comment  ( constructively that is)
Tension is easy to measure in a belt when access is available to the points of external force being applied. If we could somehow measure  the side load in the camshaft bearings for example and then divide it in two we would have a figure of belt tension. This we cannot do, so other methods are needed. The approach is based on the elastic properties of the belt material. As the tension is increased the belt stretches. So if at a given tension the overall length og the belt is X mm. then if we increase the tension of the belt by adding a given extra belt length path then we can calculate the original tension ( assuming linearity in belt length to tension over this range)
So if we increase the distance by which the belt must travel , by such means as the 'wiggle' in the Official Tool, by a small percentage, dt (%), then the tension will increase by dt as well. Knowing dt will enable us to know the value of T, the actual belt tension. Of course in practice the actual values of tension and wiggle are replaced by easy read number scale ( 0 to 10 in this case)

All we need is a tool which will 'deform' the straight section of belt by a fixed amount and then measure the force needed.
Introducing a wiggle in the belt ( or a twist as per the finger thumb method)
has its results also dependant on certain belt material properties, properties that can vary with age, temperature, and possibly oil contamination and humidity. It is probably best to assume the Official Tool and all derivatives thereof are most accurate on a new belt


Well I did several things :-)
First I ran a a finger thumb method test, setting tension by the Finger thumb 90 degree test method several times and recording the actual tensions with the Official Tool Results, not too accurate, ranging from 3.0 to 6.3 ( Book figure of 4.5 is correct). Whilst this method puts the tension ion the correct order of magnitude it is NOT at all accurate ( no surprises here)

Next I made a little device with telescopic tubes and springs and measuring rule, the idea was that by applying a known pressure I could adjust the tension such as to allow the belt to deflect a known amount After an hour of welding, grinding and making this thing, it was only a
little more accurate thew the finger thumb method. Results of same test range from 3.6 to 5.8, NO ***** good

Not to give up and realizing that the problem was not with the method but with the accuracy with which reading can be made I decided to go for a variation of the Thumb twist method I cut a section of clear acrylic about 1.5 inches wide and I guess 8 inches long ( just fits between belt at air pump and the radiator. I cut a slot so it would fit snuggly between the slots on the belt. then I set the tension with the Official Tool to 4.5. Sitting this acrylic on the belt in place of the Tool, it twisted the belt by its own weight about 1.2 inches down from the horizontal. I then ground the far end of the acrylic to 1.2 inches. Now, by simply setting the level and marking with tape on the radiator, all I have to do is to slide my acrylic tool on the belt and adjust the tension until the top edge of the acrylic is in line with the level mark. Its own weight drops it that much. This is incredibly accurate, simple and cheap. I found repeatability excellent. Setting up wit this tool gave results of 4.4, to 4.6 every time, in fact it may be the Official Tool that is not very repeatable.

So, if anyone wants more detail, measurements or whatever, just ask.
This is a cheap easy accurate method to set tension, with greater readability than the dial gauge Official Tool Anyone see any problems with this method? I found the belt twist to be approx 10 degrees, so belt properties are kept to a minimum.



Also what exactly is the official tool measuring,  tension around the whole path of the belt or just deflection between 2 points on that side only?

Here we go again , the tensioner directly tensions the timing belt from the crankshaft to the passenger side camshaft drive pulley . The rest of the
belt is running over toothed gears , the tension changes as each camshaft rotates compressing and releasing valve springs . This is why the engine
must be at TDC to measure tension . The crankshaft gear is pulling down from the drivers camgear and turns the oil pump gear along the way . The farther away from the crankgear as you follow the belt up around the oil pump to the drivers cam around the water pump pulley to the passenger cam around the tension pulley then back to the crank pulley the more work the belt must perform ; the load accumulates . This is why the drivers side camshaft is the one that breaks when the belt is too tight , the passenger camgear only has the load of turning that camshaft the drivers cam has the load of turning both camshafts and the water pump as the belt is pulled down toward the crankshaft . Which also brings up the warning never to turn the belt backwards by turning the crankshaft backwards . The tensioner is designed to carry only the load of the loose side of the belt and not all the force needed to rotate the camshafts . Turning backwards can damage the cup shaped bi-metallic washers in the tensioner .  The tension at the water pump pulley driven by the smooth back side of the timing belt varies depending on the alternate pushing and pulling affect of compressing and releasing valve springs on each camshaft . The belt acts much like a bicycle chain with half of it tight the other half just going along for the ride . The fact that it is a toothed belt running on toothed gears means the length of the belt from the crank gear all the way to the passenger camgear cannot change without jumping teeth , the tensioner is playing with the part from the passenger cam to the crank . And most significantly we are measuring the tension of a non- running engine , I believe that when running there is little or no tension at the tensioner examining the idler pulley you will note that it does not touch the belt but is needed to keep the belt from hitting it's self as it runs up to the passenger cam then back down to the water pump . For the belt to hit it must be loose . The later style tensioners have an internal baffle / damper to control flutter to keep the tension roller in contact with the belt to keep the warning light from coming on . Has anyone run their engine with out any covers and observed what the belt is doing at higher rpm ?? maybe use a timing light for the stop motion to better see deflection and cyclical motions, please do not drive on the street this way.
Jim Bailey
928 International






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