Longer belt and MMI 55 amp alternator

[rigspelt]

MMI supplies a 26" belt with the new 55 amp alternator, which is longer than the belt used for the Motorola 35 amp (25"). The new alternator comes with a pulley sized for the Atomic 4 with the longer belt (pulley diameter is about 2-3/4" as I recall, or maybe 2-7/8"). I am installing the Indigo FWC system, which means increasing the alternator belt length to 28" to drive both the alternator and the seawater pump in the Indigo FWC setup. I heard that a smaller alternator pulley (about 2-1/4") might drive the alternator more efficiently when using a longer belt. I guess the easiest thing to do is give the alternator as supplied a try next summer with the 28" belt, and measure its amperage/voltage output before thinking about whether to change the pulley. Thoughts?

[msauntry]

Devil's advocate here... If you needed more charging, the smaller diameter pulley would have less surface area contacting the belt and so it might slip under heavy load. Tightening it to reduce slippage could damage the bearings as well.

[sastanley]

it shouldn't matter???

rigs...I am no mechanical engineer, but the length of the belt should make no difference...the size of the pulleys at the A-4 accessory drive & on the alternator will determine the revolutions, whether the belt is 26" long or 100" long. My point is the addition of the FWC kit and the 3" longer belt, should not alter the alternator's output...only the pulley size, which you also allude too.

I agree with micah too...a belt getting a 180 degree turn on a pulley will get more bite and slip less than one getting only a 90 degree turn. This effect is exacerbated by a smaller pulley. We have proven this theory with ratchet blocks and spinnaker sheets. We've found that sometimes it is better to put the ratchet at the back of the boat where the line makes almost a 180 degree turn thru the block and takes more load off the trimmer and holds better, instead using the forward block, where it gets only a 65 or 70 degree turn as it heads to the spin trimmer somewhere usually forward of the cockpit.

[tenders]

> measure its amperage/voltage output before thinking about whether to change the pulley

This isn't going to tell you except in an extreme case. A squealing or slipping belt, or having to put unnaturally high tension on the belt to keep it from squealing or slipping, will tell you.

[nickmerc]

I am a mech engr and Micha is correct. You want as much engagement as possible (in this situation) between the alt pulley and the belt, by this I mean linear distance, since the belt uses friction to drive the pulley. And yes, if you over tension the belt you will wear our your bearings faster on not only the alternator, but everything else on that belt.

[rigspelt]

Edumacating myself on alternator belts and pulleys

For other non-mechanics out there, I spent some time prowling the Internet looking for what I hope is good first principles info on alternator belts/pulleys/alignment. Complex topic, and this is far from definitive. I did not find much good stuff on the net, actually. Some of these notes are results my thought experiments, so could be wrong.

Pulley alignment on all axes is fairly critical. A little off-line rubbing leads to worn belts, broken belts, squeaking, worn bearings, loss of efficiency, and other nasties.

Misalignment signs: belt dust on nearby parts, squealing, low alternator output, premature belt failures, worn belts.

Pulley size determines alternator spin rate, which is important. Too slow: not putting out enough. Too fast: not good. Alternators should be rated by designed RPM, and then the user can find the right pulley size to achieve that RPM range for a given engine application.

Pulley ratio = crank pulley diameter / alternator pulley diameter.

Pulley = sheave.

More amp output = more horsepower drain on the engine. One site said 1 hp/22.5 amps x pulley ratio. Lower pulley ratio better in terms of horsepower drain, but too low results in loss of amp output. Consider amps required at idle RPM.

Smaller pulley = lower belt contact surface, which is OK for low output, but slips in high output setups.

Things to consider if pulleys not aligning:
1. Adjust existing pulleys, if possible.
2. Adjust existing brackets on their bolts, if possible.
3. Modify the bracket.
4. Wrong pulley on alternator for application.
5. Wrong alternator casing for application.

V-belts, invented apparently in 1917, are more foregiving of misalignment (to a point), naturally pull into the groove, and pull on their "V" sides not bottom.

V belts can come with or without cogs. Cogs improve flex, but decrease pulley-belt surface area. X = cogged.

Belt cross-section shape is very important. Belts must fit well into the pulleys' grooves on the right angles, for example. More surface area = more pull, which is good and bad. There is a sweet spot for a given application.

Belt length is still a mystery to me. I know that belts are measured in inches of circumference, but for a given application, there are mechanical issues to consider for belt length that remain a black box. It is tempting to simply find the right belt length for a given arrangement, but someone like me who does not understand the mechanics could inadvertently pick the wrong belt length.

Belt length changes the angle of drive relative to the pulley centers. Long = more obtuse angle, but more belt/pulley engagement. Presumably there is a sweet spot for that factor, and disadvantages to the wrong pulling angles/belt lengths. Longer belts also have more opportunity for stretch and slap during operation, reducing efficiency.

Two-pulley system: shorter belt length decreases pulley surface contact, significant when the pulley diameters are more different, and when distances very short.
Three-pulley system: longer belt length still increases pulley contact surfaces on all pulleys.

Belt length alone should not change RPM (intuitively).

There are online calculators to estimate belt lengths given all the variables: pulley sizes, pulley-pulley center distances, angles of pull, etc.

References:
Several websites, but see also:

http://en.wikipedia.org/wiki/Belt_(mechanical)

http://mechanical-design-handbook.blogspot.com/2007/09/v-belt-drive-design.html