Slipping Vee belt

[John MCParticipant @johnmc39344]

I have a small slotting machine that has always suffered from a slipping Vee belt.  The ram stalls on not that heavy a cut while the motor continues to run.

I have replaced it with a new belt, no different.  I’ve tensioned the belt, its rather tight now, no difference between correct and overly tight tension.

I believe “Fullers Earth” was used to encourage belts to grip.  Is it worth trying or is there a modern equivalent I can try?

[duncan webster 1Participant @duncanwebster1]

Is the belt bottoming in the groove.  If so it will slip no matter what

[HollowpointParticipant @hollowpoint]

Are the pulleys worn? There’s not much you can do other than replace them if so.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi I agree with Duncan, and if it is on the bottom of the Vee, it’s either the pulley is badly worn, or the wrong size Vee section, or Vee section belt.

Regards Nick.

[Simon Williams 3Participant @simonwilliams3]

+1 for wot Duncan advises above,  Vee pullies don’t slip significantly except under obvious and significant provocation. Non slip additives just make a mess, so there is something fundamental amiss with the geometry of the set up.

Is the bottom of the pulley groove polished?  Possibly with black rubber skid marks,  If so the belt is bottoming out in the pulley groove and is either the wrong profile (size) or one (or both) of the pullies isn’t right.  As above, if so it’ll always slip no matter how tight you make it.

You can run into trouble with a belt drive where one of the pullies is way too small a radius of curvature for the vee belt profile, but usually this is about length of service rather than slippage.

Is the outer surface of the belt you are using below the outer diameter of the pulley cheeks?  If so this could suggest that the vee belt is too small a profile for the pulley profile.

Yes the pullies could be worn but this would be obvious and evidenced by the running surface of the pully not being a flat even wedge shape.

[Clive FosterParticipant @clivefoster55965]

As per previous replies verify the pulley profile by taking a mould so it’s easier to measure. If worn there may be enough meat to bring things back to the right shape but touch smaller. Remember that the pully profile varies slightly with diameter. This is important on well loaded smaller pulleys. The standard profile will work but adjusting as per book usefully improves drive without over tightening. If you find you need to run classic Vee belts very tight it’s always worth checking the profile. Has to be said that the common die cast variety tend more towards “close will be OK”, as it generally is, than “accurate to book” profile.

Cogged belts often give a better drive, especially on smaller pulleys. The minimum size of pulley needed for full power transmission via Classic Vee belts is, to many eyes, surprisingly large. With a slotter the power requirements are defined by the jolt at the start of the cut so often more drive capability than the motor power implies will be needed so things don’t slip. Regrettably it’s not uncommon for machines to be a bit under sized in the pulley department by design so wear effects how up earlier than you’d ideally like. Most likely on smaller and less expensive breeds where space and cost considerations are important.

If you do have to take the nuclear option of replacing pulleys consider switching to poly vee belts. Better power transmission and a touch more inherent resilience than classic Vee belts. Helps covering the jolt at the start of the cut. If you are stuck with special to machine pulleys built into a larger component its usually possible to machine the old Vee pulley down into poly vee form. Very easily done if make up a stop system to control the spacing and depth of cut. Worst part of converting to polly vee is figuring the diameters for a multi step set up so you don’t have to mess with the tension when changing speeds.

Clive

[john fletcher 1Participant @johnfletcher1]

First of all ensure the belt is not bottoming. Following an article in MEW I made a clutch for an Myford Super 7 lathe, the clutch initially it slipped. I went to a local music shop bought a stick of Violin bow string Wax  ??? which I crushed into a powder and then strickled some of it between the clutch faces. That was 10 / 15 years ago.   John

[roy entwistleParticipant @royentwistle24699]

John   Your violin bow string wax is actully called Rosin

Roy

[Nigel Graham 2Participant @nigelgraham2]

It’s also important that the belt is not too tight. Once set correctly it should be possible to depress the centre of the belt a little, maybe half an inch, by light finger pressure.

Too much tension overloads the bearings.

[Andrew CrowParticipant @andrewcrow91475]

If the pulleys and belts are OK there are number of aerosol sprays available on the Internet from about a fiver upwards, also useful on motor vehicle drive belts for stopping squeaking and improving drive.

[cedric 1Participant @cedric]

<p style=”text-align: left;”>Any car parts store should have belt grip spray.</p>
If the pulleys have a step worn in the V, they can often be remachined to get rid of it and restore grip.

[noel shelleyParticipant @noelshelley55608]

IF a V belt slips then there is a problem ! Wrong belt section,or a worn belt. Badly worn pulley at one end or the other. There are many different sections and it is important to get the right one. If the belt section is too small it will only pull on the narrow bottom and slip. Belt spray won’t help this. A section and M are NOT the same. For small pulleys with a low wrap angle a notched/ cogged belt can help.  Good luck.  Noel.

[SillyOldDufferModerator @sillyoldduffer]

Is the slotting machine OK?  Maybe part of the mechanism is bent.  Test for binding by turning it over by hand.  a correctly fitted belt might slip when a cutting load is added if the machine is sticking somewhere during a cycle.

Or, is the belt the correct profile for the pulley?

Dave

 

[John MCParticipant @johnmc39344]

Thanks for the replies.  The first thing I checked before replacing one of the belts was for bottoming in the pulley.  Plenty of clearance between inner face of the belt and pulley groove bottom.  The pulleys are in good condition, smooth surfaces and the correct angle.  Belt size is correct, “A” size.   No sign of oil contamination.

Dave (SOD), good point, I’ve slipped both drive belts off, everything moves freely.   Its the second belt in the drive train slipping, so it’s transmitting higher torque than the first belt.  The angle of lap is not that good on the small pulley.  Maybe a better proportioned drive?

Or, convert to Poly-vee’s as Clive suggests?  I’ve done this with a number of machines over the years with great success.  Not something I want to do at the moment, plenty of other work on the go.

I’ve just ordered a can of spray on belt gripping stuff in the hope it helps, even temporarily, so as to get the current project advanced.

[John MCParticipant @johnmc39344]

To finish this thread off, the can of spray to stop the belt slipping was delivered so into the workshop to try it.  I was sceptical, but it has helped to the point that the belt does not slip.  It might slip with a very heavy cut, not going to try it and risk breaking the tool!

The photo shows the job, cutting splines into the bore of a (4) gear blank(s).  The gears will, in due course, find their way into a motorcycle gearbox.  The grooves between the “teeth” of the splines are 5mm wide by 1.5mm deep.  The splines locate on the splines major diameter, so a tricky grinding set up to get the tool the right size.

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