Flat Belt slippage

Ive recently finished a low effort cosmetic restoration on an old camel back drill, your standard scrap and paint affair. After getting it all assembled, this thing is nearly worthless with the belt slip it has. Both power and hand feed will stall the spindle with any significant down force. As in, a 1/4" drill will slip the belt. Nothing is binding, all the bearings are oiled, power is being supplied by a 1.5hp motor with the lower cone pulley running around 250 rpm.

The belt is brand new from McMaster, 3 ply super grip ( not leather ) Tension, if I take much more out it will be a banjo string. Also, all the pulleys are clean, smooth, and have been liberally coated with belt dressing.

When this machine was first brought home, it would plow a 7/8 bit through a plate without even a grunt, the only change being the new belt that replaced an old rubber conveyor belt someone hacked onto the poor thing.


Any thoughts?

Check the back gearing on that camelback drill. You do not state the manufacturer of the drill press. Back gearing was used on many camelback drills to give an extra slow range of drilling speeds. Different manufacturers of camelback drills used different designs of back gearing. It is the same principal of back gearing as is used on many "cone drive" (headstocks driven by belting and cone pulleys) lathes. Some makers of camelback drills used jaw (aka "dog") clutches to engage/disengage the back gearing. If you have the back gear clutches caught between "direct" (back gearing disengaged, cone pulley drives the spindle gearing directly) vs "back gearing 'in' (engaged, giving deep gear reduction), you will wind up with the spindle turning when there is no load on it. When you put load on the spindle such as drilling a hole, the spindle will stall.

There are other designs of back gearing used by the various manufacturers to get extra-deep gear reduction to the spindle. I believe one maker used a set of planetary gears adjacent to the step-cone pulley on the top shaft (horizontal shaft which has a bevel pinion gear to drive the spindle). The planetary reduction units often used a locking pin. If the pin was "in", the top cone pulley drove the top shaft/spindle directly with no further reduction. If the pin was "out", the planetary unit was engaged and added deep gear reduction between the top cone pulley and the spindle. If that pin was not properly positioned (neither "in" nor "out"), the top cone pulley will freewheel on the top shaft. Oil or grease in the gearing will provide enough drag to cause the spindle to turn, but when real load is applied, the spindle will stall.

My first guess is you do not have the back gears fully engaged nor fully disengaged. The other possibility, and no disrespect intended here, is that you left out a shaft key in your reassembly of that drill. If you left out a shaft key on something like a step cone pulley, and tightened the setscrew in that pulley, the drill spindle will turn, but will stall under light loads. The setscrew is providing enough friction to transmit enough power to turn the spindle, but will slip under any real load. If you missed installing a shaft key, and are relying on a setscrew to transmit the torque, it won't happen and you will wind up with a chewed-up shaft.

I tend to think your problem lies in the back gearing. To get the back gearing to engage or disengage on these older drills, particularly the ones with the jaw or dog clutches, you have to pull the belting by hand and work the back gear lever until the jaws on the dog clutch line up and fully engage. Similarly, if the drill uses a locking pin for pinning the bull gear to the top cone pulley (as is done on lathes) you have to make sure that pin is either fully "in" or "out". If the pin is "in" (locking the bull gear to the cone pulley), the drill is in "direct", the top cone pulley will drive the spindle thru the bevel gears. If the pin is "out", the cone pulley will freewheel on the top shaft until you shift the back gears into engagement. Do NOT attempt to work or shift the back gearing, whether it uses dog clutches, or moves a set of gears in and out of mesh, with the drill under power. Do not expect the back gearing to go smoothly in or out of engagement. Pulling the drill's top shaft over by hand while working the back gearing is usually what it takes. My guess is you moved the back gear lever and thought you had things engaged, only things are caught between back gears "in" or "out".

This is a Dexter drill, I have so far been unable to find any info on it.The belt is slipping on the top pulley, and I have not been into the topside yet. From the look of it, this one simply has a bolt holding the back gearing in direct range, removing the bolt and shifting the gear lever just rotates the gears on an eccentric into mesh.

I have the same belt on my Garvin vertical mill and it does not slip but it is tight. For leather belting the tension that is recommended is 600 pounds of tension per 1 square inch of cross section so a 1/4" by 4" belt should have 600 pounds of pull on it. I would assume that the rubber belt would need less tension but as a guess 300 pounds would be good. I think you just need more tension than you think.

enginebill said:

I think you just need more tension than you think.

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Either that or just go back to the one that worked ...

When I was a young apprentice in 1960's and we still had plenty of line shaft machinery we used to throw lumps of " Shellac" , like large brown shiny sugar cubes, into the " nip " between the pulley and the belt whilst they were running. That worked a treat. Later on you could get an aerosol spray called something like " Belt Grip " that also worked pretty well.

Regards Tyrone.

We used to have a spray can of belt-grip stuff that looked like tar..now it seems that is not on the market any more.

That spindle should turn by hand, or hand pulling the belt when not drilling or something is wrong

We had a Champion camel back at work and we had problems with the flat belt slipping....Use to keep a can of belt dressing near the unit..The stuff was nasty and black but it made the belt grip...I have a 21" Buffalo in my home shop which I screwed a large shop made steel v belt pulley to the cone pulley on top... I use a large V belt to run mine....I have a 2hp three phase motor running a Dodge m37 transmission on the bottom with a cast iron v pulley on it which drives the upper pulley....No slipping flat belt now.. Ramsay 1

Thanks to all for the advice, I cut another 1.5" out of the belt and all seems ok. Tighter than I would think it would be, I have to shift the belt with and old axe handle but it will still roll over by hand.

The rule with flat belts is that you start with enough tension that when the "loose" side is getting down toward zero tension, you have more than enough tight side tension to transmit the power.

As the belt transmits more power, there is stretch in the belt. When the belt comes off the pulley, the stretch is maximum right where it leaves the ulley. Back farther on the pulley, the tension is partly taken up by friction on the pulley, and the stretch is less. The stretch causes the belt to "creep" on the pulley. That stretch also occurs on the slack side, the tension gets taken up from near zero where the slack side comes around (at near max power) to be max as the belt leaves. That creeping is what causes the belt to "talk" as power transmitted increases.

At some point between the slack and tight sides, there is an area where the tension does not (yet) cause creeping, the tension is not enough to produce sufficient stretch.

With more power, the belt stretch and movement increases, and tends to move back around toward the slack side. So, when the power transmitted is enough that the belt is still stretching and creeping back where the slack side is coming onto the pulley, you get slippage.

By increasing the zero speed tension, you increase the power (and so the tight side tension) at which the creep point gets around to where the belt slips. That can seem pretty darn tight. And it can be enough to actually stretch the belt permanently.

The higher power for double layer belts is because you can put more tension on them without starting to stretch them permanently.

Switch to an auto serpentine belt. Go to the South Bend section of this site and search Serpentine belt for ideas.

I think the belt dressing we used at work years ago was "cling surface"...It was in a yellow can with a spout on it.. Nasty black stuff that set up with a sticky, tacky surface on the pulleys and belts....I still have some I use once in awhile on my Southbend lathe but not often.. Ramsay 1

IMO old flat belts should be able to produce just as much power as V-belts of corresponding sizes, but they do require more maintenance. Tension is probably the biggest thing. It's the same with V-belts, but most people don't realize it because V-belts don't stretch as much as many flat-belts so you can get the belt up to tension with less adjustment. That's why people have so much luck with more modern synthetic flat belts. Stretch aside, the only thing that separates flat belts and V-belts is that a V-belts shape makes it more compact and is more self-aligning than a flat belt.

In many leather belt applications, it is advised to have 2 or my ply belts to reduce the stretch. Belt dressing helps a lot, but I've always seen it as just a way to "rubberize" the surface of the belt, not glue it to the pulleys. You can still buy a few different varieties of belt dressing, but I tend to use some home-cooked stuff on my leather belts. It's a mix of Rosin, Pine Tar, Pumice, Castor Oil, Tallow, and Asphaltum. Anything sticky helps, but you want it to spread on easily and stay tacky. I rarely use belt dressing on V-belts, or other rubber type belts, because they should have that quality built in. If they are slipping they are either loose or worn out.

Speaking of flat belts.....


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swatkins said:

Speaking of flat belts.....


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