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flat belt tensioning

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
The last flat belt machine I worked on, the belts were what they were. There was no tightening them beyond cutting and relacing. I did that on the main drive belt of that machine and after it was nice and tight and didn't slip.

The current machine I'm working on has an electric motor on a factory mount several feet above a jackshaft. That belt is about 3" wide. Assuming the jackshaft is fixed, the motor belt can be tightened by adding shims between the motor and its mount.

The machine's main drive belt goes from the jackshaft up to a top shaft. There isn't any way to tighten that belt. I could loosen it if necessary by shimming under the jackshaft base, but I can't really tighten it.

I have a collection of vintage fast/loose pulleys about the same width as the main drive belt (probably 1-7/8"). I'm considering cobbling up a base for a rotating arm with one of those pulleys mounted to the end and a weight on the other end which would force the pulley against the outside of the belt, effectively tightening it. If it were simple to pin the lever so the pulley didn't bear on the main drive belt, that would effectively remove most of the tension from the belt which you're supposed to do when you're not using the machine.

I have never seen a tensioning device like this on a flat belt drive. Is there a reason I shouldn't do this?

metalmagpie
 
I have just acquired an old Buffalo Forge 21" camelback and have realized the same issue... there is no method of tightening the flat belt between the jackshaft and main gearshaft. I am considering doing exactly what you are suggesting...

I won't get to mine for a while, I have another project in process, but I'm looking forward to seeing what you do with yours...

-Bear
 
Just make sure you put it on the slack side of the belt. An idler pulley is not uncommon on flat belt drives as a clutch. No reason not to use on as a tensioner apart from the additional complexity.
 
No reason not to add an idler. You can also raise the lower cone bracket on shims, shorten the existing belt, and then remove shims as the belt stretches. I did that on my Royersford drill, and it worked well, though mine had a different motor mount that used vee belts and included belt tension adjustment.

Sent from my SM-G950U using Tapatalk
 
The serpentine belts used on vehicles are essentially flat belts and all use spring-loaded tensioners so no, there is nothing wrong with adding a tensioner as long as it's on the slack side. Counter weights are actually superior to springs which is why they are used on many critical applications including tensioning cables on ski lifts.

If you rig the tensioner so it can accommodate the entire width of the step pulleys you can leave the normal tension somewhat loose to make changing speeds easier.
 
Put a steel tape around the pulleys and get an accurate measurement of the belt length. Then subtract 1/10" per foot of length and cut your belt to that length and lace. Your belt will be the proper tension then and you will only need to shorten occasionally when the belt stretches and you won't need a tensioner. You can run the belt off the pulley by hand if you want to relieve the tension.
 
Metalmagpie:

Idlers to tension flat belting were used in a number of applications. A very common one was a weighted idler to tension the flat belt driving steam engine governors. This idler had a dual purpose. Aside from tensioning the governor drive belt, the 'hub' of the idler had a cam called a 'knock off cam'. If the governor belt broke, the engine could run away (dangerous, if not destructive overspeeding). The knockoff cam rotated with the idler arm and worked the throttle valve in the governor to shut off steam.

Ingersoll Rand described what they called a 'short center' flat belt drive for their bigger recip compressors. This allowed the motor to be mounted close to the compressor, and the idler was used to increase angle of wrap of the belt on the motor (relatively small pulley) and on the compressor flywheel.

My first exposure to idlers on flat belt drives was at Brooklyn Technical HS in 1964. The first shop course we took was wood patternmaking. The shop class rooms dated to the 1920's. In our pattern shop classroom was a Tannewitz band saw with flat belt drive. There was a weighted idler pulley to maintain belt tension, as well as get more angle of wrap since the motor sat at floor level, quite close to the bandsaw. When the motor was started, the bandsaw wound up to speed, and that idler hopped up and down on the belting until load was put on the saw.

At Hanford Mills Museum, there are numerous flat belts and line shafts as well as woodworking machinery driven off the line shafts. For the most part, the belting is tensioned by taking pieces out as needed and relacing (installing new hooks). There is one really wide belt that transmits most of the power in the mill. This belt is on fairly close centers and was always a problem to maintain tension. One of the mechanics who worked at the Mill made an idler pulley, and used a scissors jack from a car to adjust the idler and tension the belt. Backwoods fix, but it does work.

In shops and mills with a lot of flat belting coming off line shafts and counter shafts, maintaining belt tension was always an issue. In the shops and mills with a lot of belting, a cart with the belt cutter (shear) and lacing machine and assortment of lacing hooks and pin material was often used. The cart was knocked together out of wood, and was rolled to wherever a belt had to be relaced.

Enginebill is correct a bout running the belt off by hand to relieve the tension. When I was a student at Brooklyn Tech, we still had many lineshaft driven machine tools. Similarly, as I began working part time and summers in a machine shop, there were still some line shaft driven portions of the shop. At 'Tech, the belts were taken down over holidays and vacation time. This was to prevent the belts taking a permanent stretch. In the machine shop, over a summer weekend with humidity, the fear was the belts would permanently stretch, or at least be too slack to transmit power without slippage come Monday mornings. We used to run the belts off the pulleys each Friday when work was done. The belts were hung adjacent to the line shaft pulleys on heavy wire hooks. This was to keep the belts from soaking up oil that might have been in the grunge on the line shafting. Coffee cans or similar hung on wire under some of the lineshaft bearings which did not have the cast iron troughs below them. These cans of dripped oil had to be handed down and emptied into a bucket which went into a used oil drum. Come Monday morning, we had the job of putting the belts back up on the pulleys. Several of us worked at it. With no formal 'lock out/tag out'. the foreman would place himself bodily in front of the motor starters for the line shaft drive motors. One guy would be heaving on the spokes of the big lineshaft pulley nearest the drive motor. A couple more of us would climb like a squirrels and work the belts onto the pulleys as the lineshaft was heaved around. We came in a half hour early for this detail, and oiling the lineshafting was also a part of it.

With all of this, we still would start up the shop and as someone hogged into a cut, a slipping belt was occasionally encountered. Out came the lacing cart. The cart was ancient (or so it appeared to me as a kid). It was battered and covered with belt cement from cemented splices, and had a lever operated shear for cutting and squaring the ends of the belting, as well as the lacing machine. There were also some clamps and a drawknife for 'skiving' belting to be splced by gluing. I never saw a glued belt splice made up. Lacing hooks were about all I ever saw in use. The slipping belt was shortened and relaced, and put back up without stopping the line shafting. It was run onto the pulleys. At Brooklyn Tech HS, our shop teachers showed us how to shift a belt on step cone pulleys on running machine tools. They used the sides of their hands, and had the belts on cone drive lathes seeming walking from step to step of the pulleys with no problem. We kids were warned not to use our hands, but to use the smooth side of a wrench, or a stick. An 'engineer pattern' wrench with an oval cross section to its handle, laid with the flat side against the edge of the belt made an ideal belt shifter.

It's a long time ago that all of this was happening. I was fortunate to get a belt cutter (shear) in a lot of junk my Dad bought at an auction in 1958. I got a belt lacing machine and a mess of lacing hooks on paper carding when we cleaned out an old hydroelectric plant. In 1921 when the plant was built, the governors were flat belt driven from the turbine shafts. In 1959, the plant was modernized with Woodward ballhead hydraulic governors using a synchronous motor drive and permanent magnet generator. This eliminated the need for flat belting. The flat belting stuff was stored on a shelf and forgotten about, along with glass globes for vapor proof lighting that had been painted for WWII 'blackout' lighting. I grabbed the belting stuff when the contents of that store-room went into the dumpster. I had to explain what 'blackout lighting' was to people who wondered why there were so many light fixture globes painted black with only a small slit left unpainted. There was a lot of old stuff in that store-room which I had to explain the usage of and determine if it were to be kept or junked. I got a lifetime supply of belt lacing hooks, aside from the lacing machine, so made out handily. I also got a Pexto drawknife, which I suspect may have been used to skive the belts for glued splices. Never found any clamps for glued splices, so am guessing about the purpose of the drawknife.
 
Here is a picture of my flat belt drive Hardinge lathe. I built the bench and drive myself, using a Rivett 3-speed counter operated by foot pedals. The three upper belts are leather with Clipper lacing and have not needed to be adjusted in decades of use. The lower belt is Habasit, a synthetic fabric that is cemented after running through the headstock. The idler pulley is paper and as wide as the cone pulleys. I clamp it at the arm pivot rather than depend upon its weight to maintain tension.

Larry

59 flat belt 2.JPG
 
The serpentine belts used on vehicles are essentially flat belts and all use spring-loaded tensioners so no, there is nothing wrong with adding a tensioner as long as it's on the slack side. Counter weights are actually superior to springs which is why they are used on many critical applications including tensioning cables on ski lifts.

If you rig the tensioner so it can accommodate the entire width of the step pulleys you can leave the normal tension somewhat loose to make changing speeds easier.

Serpentine belts are grooved on one side (sometimes both sides are grooved) and flat belt pulleys are generally crowned to keep the belt tracking correctly.

No comparison in my opinion
 
Serpentine belts are grooved on one side (sometimes both sides are grooved) and flat belt pulleys are generally crowned to keep the belt tracking correctly.

No comparison in my opinion


They ARE quite different, at least in terms of what keeps them on the pulleys, but it doesn't affect use of an idler for tensioning. Idlers work regardless.
 
They ARE quite different, at least in terms of what keeps them on the pulleys, but it doesn't affect use of an idler for tensioning. Idlers work regardless.

I didn’t comment on the use of tensioners, only the comparison between serpentine and flat belts.
 
I have just acquired an old Buffalo Forge 21" camelback and have realized the same issue... there is no method of tightening the flat belt between the jackshaft and main gearshaft. I am considering doing exactly what you are suggesting...

I won't get to mine for a while, I have another project in process, but I'm looking forward to seeing what you do with yours...

-Bear

I have a 21 inch Buffalo that I converted to v belt drive years ago.. I made a large v pulley from steel and screwed to the top pulley then used another v belt pulley on the back of a Dodge M37 transmission driven by a three horsepower Allis Chalmers three phase motor... Cheers from Louisiana.. Ramsay 1:)
 

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My M37 still has its transmission...;) Had it out this morning as a matter of fact. Happy Independence Day.
 

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My M37 still has its transmission...;) Had it out this morning as a matter of fact. Happy Independence Day.

Hey Nice Dodge! I drove one for 20 years and it still runs and is being used for a crane lol......Here is mine being used to hang steel for my shop.. This one is a 1952 model .. I had an extra tranny and transfer case from some that I scrapped...Cheers from Louisiana... Ramsay 1:)
 

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A very simple belt tensioner Design is that use on old garden tractors being just a couple bars and a pully shaft. pushing In with bet direction just ahead on the next pully they increase the amount of belt on the pully and so reduce slipping.
Plus such a design can act as a clutch.
 
A very simple belt tensioner Design is that use on old garden tractors being just a couple bars and a pully shaft. pushing In with bet direction just ahead on the next pully they increase the amount of belt on the pully and so reduce slipping.
Plus such a design can act as a clutch.

Michiganbuck, you've forgotten to take your medicine again. :-)

Can you please translate?
 
Older small tractors were belt driven and a simple swing arm with having a pulley would tighten the belt enough to engage power. Often the lever arm would swing past the pivot center to a stop so that would facilitate locking the swing arm in place.

We had a fellow here in Detroit back in the late 60s Or so who built about 5 prototype belt drive simple tractors about the size of a ford (9n. I saw the ad in the paper announcing this tractor for third-world countries.

A few years I saw the auction of the works. Then one day out looking at a tractor for sale I met a fellow who bought one at auction and installed a small three-cylinder diesel. It was just a pulling tractor but except for replacing belts, it was/seemed very capable and low cost to maintain.
 








 
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