Pulley construction query

A picture of a random collection of pulleys

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brass and aluminium for use with rubber drive bands in tape recorders, VCRs etc. They all have a small groove at the root of the V section of the pulley. Anyone know the function or reason for this groove?. All I can think of is it may avoid an aquaplaning type possibility of air getting trapped under the rubber band snd so wow/flutter possibiliy.

-- General electronic repairs, most things repaired, other than TVs and PCs

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Diverse Devices, Southampton, England

Reply to
N_Cook
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A V-belt won't work if it bottoms in the groove. This is just an undercut to ensure there is no radius at the bottom of the contact faces.

Reply to
newshound

Often the drive belts are square cross section rather than round and sit neatly in the root of the pulley slot. Maybe this has some bearing on the design?

Reply to
Den

Like I said, except that these pulleys all seem to be for square belts. For these to work, the root radius of the pulley has to be smaller than the radius of the belt corners. With an undercut slot like this even perfectly square section belts will be fine.

Reply to
newshound

more likely a machining aid during manufacture. a V point tool cutting on both sides of the V is pretty well a machining nightmare. far easier to make a slot then do a single face cut out from each side to form the V.

Reply to
Stealth Pilot

The way V belts work is that they must wedge between the angled sides to grip. If the bottom of the belt touches the bottom of the pully groove, they will slip. Please refer to the Machinists Handbook for explanation. Please also note that the angles are not same for all V belts, not even for different diameters within a belt series. The angles are optimized for grip and ease of release. The big boys make these pulley with a single plunge cut, but the machine that do these are massive 30 ton lathes. The big machines can cut 6, 5/8 grooves on a single plunge in iron. The rest of us should do this in 3 operations per groove. First a plunge cut to total depth with a parting tool and the second and third operation is to use the compound set at the correct angle first one way and second the other way. No stress, no pain, no chatter. Piece of cake. Steve

Reply to
Steve Lusardi

I can't see any real reason for it especially in your applications.

Reply to
Meat Plow

The groove is just to make them harder to clean, of course.

The light-duty belts used in these small mechanisms typically aren't V-belts of the common type used in larger appliances or machines. The VCR and other belts are rarely ever cord reinforced, and belt tension is low to barely adequate.

As Den mentioned, and I know Nigel already knows, the belts are often square-cut in cross section (90 degree V).

The groove will allow the belt faces to completely contact the pulley drive faces, allowing the most effective transfer of motion. The square drive belts rarely shed (but commonly glaze), but can stay completely seated in the pulley groove under varying conditions.

In addition to allowing full contact between the belts/pulleys, manufacturers will do almost anything to save raw materials (the compound or metal used for the pulleys), to save costs and to make the parts lighter.

It surprises me, the lengths that manufacturers will go to in mold making, to save materials/resin, which in most cases jepordizes the strength or mechanical integrity of a part (part weakness increases the chances of the part being non-repairable or non-reuseable).

WB ......... metalworking projects

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Reply to
Wild_Bill

The band used for these small pulleys are usually square cut rather than V section but I think you have the reason for the groove. If the square section deforms slightly to a diamond shape and a less than 90 degree corner sits in the pulley rim then you would have the same bottoming-out effect.

-- General electronic repairs, most things repaired, other than TVs and PCs

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Diverse Devices, Southampton, England

Reply to
N_Cook

Out of interest what is the advantage of square section belts - are they easier/cheaper to manufacture than O section belts????

Reply to
Den

I've always assumed that it gave the best surface area and so drive force for a given width of pulley. The contact area would remain much the same on stretching compared to a circular belt in a circular section of pulley rim and progressively less contact area on stretching. A circular belt in a V section pulley would not have much contact area. Square section belts can be made by cutting after the cyluinder is extruded but round would have to be 2-part moulded I would have thought, leaving a mould line.

-- General electronic repairs, most things repaired, other than TVs and PCs

formatting link
Diverse Devices, Southampton, England

Reply to
N_Cook

It's there because it's impossible to make a pulley wiht ZERO radius at that corner. The zero radius clearance is requuired because square belts may come with arbitrarily sharp corners, which would keep the belt from seating properly if there was any radius in the pulley.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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