Timing Belt - Is Arnnworx Gone?

[RCRead]

My '88 944's timing belt is due to be replaced this winter, so I went online to buy the Arrnworx belt tensioning tool. Unfortunately, it looks like the website is no more. Does anyone know what happened to Bruce? What's the next-best method for tensioning the timing belt?

[Eskozki]

I've replaced the belt once with no special tools. I used the two finger twist method. On the longest part of the belt, use your thumb and forefinger to twist the belt, if you can do 90 degrees it's tight enough. Clarks describes the twist like this:

At the midpoint of the longest run of the timing belt between the camshaft sprocket and crankshaft sprocket, grasp the belt between the thumb and forefinger. You won't be able to get at the exact midpoint. Somewhere near the cam belt idler pulley will be fine. Twist the belt with your thumb and forefinger only. If you can twist the belt 90 degrees that's perfect. If you can't twist it more than 45 degrees without a great deal of effort, it's too tight. If you can twist it more than 90 degrees with ease, it's too loose.

[walfreyydo]

If youre not comfortable with the twist method youll have to find a tensioner tool used. I thnk they come up from time to time on the classified forum.

[kdjones2000]

Your '88 has the spring loaded autotensioner.

Ideally you do not need the tool for the cam belt. Might want something for the balance belt however.

[djnolan]

Has anyone tried the stethescope method?

[wwdwgs]

Originally Posted by djnolan

Has anyone tried the stethescope method?

Can you explain this method?

[djnolan]

First you need an automotive stethoscope, similar to what a doctor or nurse use. They are under $10 at harbor freight.

It has a probe that you can put against the belt roller hub or stud to listen to the pitch they make when running. You will need to run the engine with the upper belt cover off to do this.

After setting the belt with the twist method, you can experiment with different belt tensions while listening to the roller. To tight and it will make an unpleasant sound. Too loose and the belt will flop.

The water pump roller is one to listen to. Also the tensionor roller on both belts.

You can also use the stethoscope to check for bad rollers.

this method would be for a car without an automatic tensioner or for the balance belt.

[wwdwgs]

djnolan, I see.

[Monkey Wrench]

the purpose of quantifying it is because one mechanic will inevitably have a slightly different feel than the next. - how important it is that it is set perfectly, Ill stay out of that.

a simple way I've seen it documented in machine setup info is to use a fish scale , then you can pull on it with for example 20 Lbs force and see the belt deflect by , lets say for example 1/4 inch.
this means every tech needs a fish scale , or one of those electronic scales to measure baggage weight may be ok. its not an expensive tool.

a more accurate way is to use a brecoflex meter, it uses the resonance of the belt to judge tension.
the way the use of that was documented was to put a paper clip on the belt, I forget but they may have added a nickel ..

-- it needs metal to detect the vibration.

so then the belt is "thwapped" ( not running) and the resonance produced can be measured. this means a more consistent measurement can be made despite variances in "feel"

this is very much like using a guitar tuner to set up a guitar , it takes away the guesswork, a pro guitar player could likely do that by ear and experience and may have reason to vary his tension, but for the beginner he is at least starting close to where he should be.

now the meters may have evolved to not need metal , that was going 20 years back for me..

and if you were to change the type of belt than maybe the frequency could be different, we were using a stock belt available through the company that documented the procedure, so no one had a different belt.. but IF the belt weight or other characteristics were different, so would be the resonance, this may transfer to differences in frequency.

The application demanded a more accurate reading than the "fish scale method" but I've seen and used that in other machine documentation.

Porsche used a belt tensioning device that was not frequency dependent, but it stands to reason that if you set a belt as accurately as possible with the stock Por$che tool, then tool, that a brecoflex meter could be used to "copy" the setting. - along with the same brand of belt.

changing belts could be akin to the guitar example, movingot a different string, different thickness, different weight , same tension, results in different frequency.

perhaps the "Porsche way" is less dependent on brand as it is not relying on resonance?

heres' an example of a modern one. I think this is what was being referred to.

https://www.brecoflex.com/products/sm5-tension-meter

a bit more than you wanted to know but this is why it may be important to use one..

now this belt is a cogged belt, there is no slippage, ever it would have to jump teeth to slip and that should never happen.

in reading the comments above about listening to the belt I wondered how much is due to the frequency of the belt itself..
In other words, independent of tension but depending upon speed and the number of cogs, there could be some resonance from that but I would think that would be mostly dependent on the speed of the belt, the number of cogs is fixed. it never slips and the number of cogs cannot vary.

what djnolan is talking about is that he has developed a sense by listening to the belt that relates to its tension, there may be weight to this. I do not think i could jump up and make a very accurate judgement that way. I think this requires some degree of practical experience. I am not saying it does not have validity. I can make sense of the twist method, I know it is an estimated way and perhaps enough?


any CNC machine needs to move its mechanical parts accurately, a ballscrew can be used for extreme accuracy, or a belt, when a belt is used it may be quite long and it is usually run by way of a stepper motor , so for example giving 1000 pulses may move the belt and any attached parts by 1 foot, this is calibrated electronically. in this example each pulse is 1/1000 of a foot. If it is for example running the positioning of a milling cutter that belt tension may be important to maintain a given level of accuracy.

the scale or amount of movement per step can be adjusted through electronic settings, that is how one adjusts the machine to move it's distances accurately.
the "offset" which is a starting point can also be fine tuned electronically, by changing settings. that is the basics of machine calibration.

often when the machine initializes it goes and looks for a position indicator, it meets a microswitch or reed sensor or similar. then the machine knows "where it is" after that it can continuously move it's parts to exacting locations. in some cases the machine can store that position in its own microchips, if so then powering it back up results in the machine remembering its position.

you can find that sort of system being used in all sorts of modern machinery.. it is very accurately repeatable. even your desktop scanner will be using this sort of technology. when you hear the brrrrr of the stepper motor what you are sensing is the pulsations actual steps, the computer remembers where it sent the motor and it ads and subtracts and keeps track of the parts that way.

often Ill find that a person is confused and things something is broken because the computer got confused, it then may go searching slowly, so its is then delivering the steps in a slower than normal pace, people then thing the machine is broken or something, its not broken, it is lost, and the computer knows that and is then often just telling it to move its parts more slowly, the result can be quite a loud noise which sounds abrasive. the computer may be programmed to go slowly to home for example, It is common to program the machine to be going slowly , because at normal speed may result in a costly collision of parts.

if the machine binds the computer may sense "lost pulses" it sent pulses which did not result in movement, then it may "know" its in trouble. in some cases it returns home after each cycle and does the math.. if it finds the pulses are becoming "lost" , then it throws up a warning, this can be for example, a mechanical bind. bad motor, a pulley came loose, etc. belt tension is a variable that can amount to inaccurate positioning and they do stretch in time. the amps each stepper motor draw can be monitored, so if a machine has mechanical problems ( bad bearing) then it may sense the draw to a stepper motor is too high and then it may be programmed to stop or throw up warnings before it reaches a point of failure.

part of my job going back 20 years or so , was to diagnose stuff like that remotely so for example I could slow the motor, change parameters that caused the error , send a new motor and belt maybe some bearings and a tech onsite and we did it remotely so it could be in Australia, or Alabama, location didn't matter much..

and in that way we could prevent failures and keep the machinery in operation. the machine could have a warning at night, Id resolve the immediate concern , keep them running by loosening parameters, making electronic adjustments, and figure out what was wrong, by morning they had parts onsite and a tech at the door.. cool technology.


that wont help you fix your Porsche it just explains why the meters like this exist. belt tension can affect the accuracy of CNC driven machinery. as we go ahead this sort of machinery is all around us, and the basics that I explained above relate to many applications.

sorry I went way off on a tangent, I thought someone may find it interesting or perhaps useful in some other application.

I don't know anything about modern cars, but I have heard of things like variable valve timing so maybe some of this is similar. in other words a modern ECU may indeed be able to control things like valve timing , using stepper motors and related technology. a modern mechanic may know a lot about all of that.

[curtisr]

Great stuff, Monkey Wrench!

The kinder, gentler folks at carpokes provides members with the .stl file for a printed tool.


https://www.planet-9.com/threads/3d-printed-944-timing-belt-tensioning-tool.253729/