Belt tension

[Sean Boyle]

I recently rough turned a couple of large cherry bowls While getting them round I noticed if I got a catch the bowl would stop but not the motor. I can adjust belt tension but how tight should it be. When I stall the bowl should it also stop the motor or is it better to let it slip.

My lathe is a General 26020 motor is mounted in base near the floor and uses a approx. 4 foot belt

 

[Mark Mandell]

Let'er Slip

Good safety measure. I have my lathe (Stubby 1000) purposely set so that the belt will slip before a dig (not that I have any ) or similar error will rip the wood out of the fixing. Especially good if you get into coring out multiple roughs from a single bowl blank.

 

[robo hippy]

Sean, I think a belt that long will slip no matter how tight you get it. I prefer to have the belt as tight as possible. I have one of the grooved ones now, and not the old standard V belts. It becomes a matter of learning to FEEL how much pressure you can apply before your belt starts to slip. One problem with slipping, if you do too much of it, you can wear slick and bumpy spots in your belt which can add vibration.

robo hippy

 

[Owen Lowe]

As to how tight, make it tight enough to not slip when applying your individualized cutting pressure. (I may apply more pressure, so may need a tighter setting; or lighter/less tightening.) It’s just downright annoying when the belt slips for no good reason.

You want to avoid more tightening than necessary because it wears the belt, motor and spindle bearings, and makes the mechanics run hotter.

 

[hockenbery]

We have a 260 with the vee belt and step pulleys.
I just press down slightly when I lock the motor.
The lathe just did a whole lot of hollowing in a class.

Lathes like the ONEWAY have an automatic shut off when it senses a catch.
The General does not have that feature. So having the belt slip is a good thing.
When you get a bad catch it is because the wood is driving onto the tool and the tool cannot escape.
Something has to give. The best thing to give is stopping the lathe.

If the lathe doesn't stop something has to break to release the tool from the wood driving onto it.

Al

 

[john lucas]

I apply enough pressure to drive it but not enough pressure to put undue stress on the motor bearings. Don't know if that's good or bad, just the way I learned to do it.

 

[dbonertz]

My lathe like Al's will shut off if it senses a catch. The belt will still slip on occasion also. I don't worry so much about the spindle bearings with over tightening the belt since I am not strong enough. I am more concerned with motor bearings as John mentioned. If you can let the weight of the motor be your tensioning and just a titch of hand pressure after that should be plenty of tension.

 

[Bill Boehme]

Technical Information About Belt Drive Systems

... I can adjust belt tension but how tight should it be. When I stall the bowl should it also stop the motor or is it better to let it slip.

The ideal situation is to not get catches. Catches are inevitable, but don't use belt tension as a way of dealing with that problem.

Here are some useful things that woodturner's need to know about belt drive systems:

• Overhung loads-- Electric motors don't like high radial loads and ideally the load would be purely torsional without a radial component. In addition, high efficiency motors have an extremely small air gap between the rotor and stator which isn't very tolerant of side loads that can eventually cause motor failure if the rotor and stator make contact. Bottom line is to minimize overhung loads as much as possible.
• Drive belts -- for most lathes, whether conventional "A" section or the newer "J" section belts (known as Micro-V, Poly-V, Multi-V, etc), they all work on the same principle of wedging friction created by running on the sidewalls. It takes very little belt tension to reach its maximum operating friction level and increasing belt tension beyond that only serves to accelerate belt wear and lower drive train efficiency. Recognizing when a belt is worn out is important -- here are some indicators:
  • As a belt gets old, it gets hard from a failure mode known as ozone cracking. Belt friction decreases which means that it will more likely to slip. Other failure modes usually show up before this becomes very evident, but an old lightly used belt could become unusable because of ozone cracking.
  • Sidewall wear is the normal wearing out process of a belt. When the belt face -- peak of the ribs (for J section) or bottom of V (for A section) -- reaches the bottom of the pulley groove(s), the belt is worn out because the sidewalls can no longer create sufficient wedging friction. It is important to recognize this and not attempt to compensate by increasing belt tension.
  • Glazing -- running with too much belt tension can glaze belt sidewalls and result in a situation that is similar to ozone cracking.
• Efficiency -- this is the ratio of power out divided by power in. A belt in good condition is elastic and is capable of storing and returning energy as the sidewalls are wedged and released. The efficiency of J section belts under proper tensioning is about 98% and about 90% for a conventional A section belt. Excessive tension will lower the efficiency considerably.
• Belt tension -- the proper tension for an A section belt is typically determined by stating the belt deflection at mid-span should be about 3/8" to 1/2" with light finger pressure. For a J section belt the deflection would be a a bit more, perhaps 1/2" to 5/8". How much pressure is "light"? I would think that if you can imagine how much force is exerted on a skew when turning a long thin finial then you're in the right neighborhood. If you get white knuckles and the veins in your neck pop out then maybe ease up on the finger pressure.

I apply enough pressure to drive it but not enough pressure to put undue stress on the motor bearings. Don't know if that's good or bad, just the way I learned to do it.

You've restored my faith in mankind, John.

 

[john lucas]

Don't know if my lathe stops if you get a catch. Never got one. Is it like hook and release when fishing. . My powermatic will not start if something like the tool rest is blocking the wood from spinning or you leave the index wheel engaged. Again never done that but I've heard rumors.

 

[odie]

There is a wooden "wedge" which I use to apply a moderate amount of tension on my lathe drive belt. I did a lot of belt changing before I converted to variable speed, but not very often anymore. It pretty much stays at the 1200rpm range since the addition of the variable speed.

I've had a couple of minor catches in recent years......not in the act of turning, but just being careless, and not paying attention to where the tool was in my hand while looking away. There is a dent in my lathe lamp......about 15yrs ago, I had a pretty big catch while hogging out the interior of a bowl. The handle of my gouge swung up and smacked the lamp shade.....no belt slip with that one. I think it would take one heck of a whopper catch to make my multiple "v" belt slip. I'm using belt dressing every couple years, as well......this makes the grip of the belt stronger.

There were a few of those really jaw-dropping catches back when I was just starting out, and I don't recall if the belt slipped, or not. I can remember a few "asteroids" whizzing through my shop back then, and thinking I was lucky to still be alive! For safety's sake, it might not be such a bad idea if someone just starting out kept a low tension on the drive belt. It's probably the exception to the rule where a newbie doesn't have at least one or two "pucker" moments!

ko

 

[Sean Boyle]

these 2 bowls were some of the larges I have turn. When I mentioned catches they weren't bad but the belt sees to be slipping quite easily

Bill

Considering it is a 20 year old lathe which was privately owned and never used a lot I know it is the original belt. I think I will replace it and leave tension unchanged to see if it makes a difference. Good news I have a spare belt that I ordered when I purchased the lathe 1.5 years ago.

I will let you know if it makes a difference

Thank you everyone of the advice

 

[Bill Boehme]

Sounds like a good plan, Sean. Belts are a lot like tires ... You've probably heard the term "dry rot" referring to old tires that may still have a lot of rubber left, but the sidewalls were badly cracked and hard. "Dry rotting" is actually ozone cracking. When ozone reacts with rubber, it gradually loses its elasticity over many years and becomes brittle. When the rubber is flexed, small cracks form and they gradually grow into cracks large enough to be seen.