A smaller alternator pulley

[Theo]

My alternator does not spin fast enough. If I put a smaller pulley on it here might be a problem with slippage. May alternator is rated at 70A how much wrap around will I need to avoid slippage?

 

N

[Giant]

Depends on the type of belt and how much tension you can feasibly get on it, surely.

 

Can you not put a bigger pulley on the other end?

[DHutch]

There are calculators.

 

What is the belt size/section? Proposed size for the smaller pulley? Percentage wrap you have at the moment?

 

 

Daniel

[David Schweizer]

It is actually the ratio between the Crankshaft pulley and the Alternatior pulley that matters, and ideally this shoukld be aproaching 3:1. which is what we have on our boat,. However to answer your question the alternator pulley on our boat isquite small at 63.5mm outside diameter, it is driven by a 184mm outside diameter crankshafty pulley and we do not suffer from slipping. The other critical issue is the width of the belt and pulleys. ours were widened out to accomodate a 13.5mm belt which seems to be adequate.

[nb Innisfree]

Twin belts make a big difference, effectively doubling the wrap.

[David Mack]

It is actually the ratio between the Crankshaft pulley and the Alternatior pulley that matters, and ideally this shoukld be aproaching 3:1. which is what we have on our boat,. However to answer your question the alternator pulley on our boat isquite small at 63.5mm outside diameter, it is driven by a 184mm outside diameter crankshafty pulley and we do not suffer from slipping. The other critical issue is the width of the belt and pulleys. ours were widened out to accomodate a 13.5mm belt which seems to be adequate.

The ratio is what matters when it comes to getting the alternator spinning fast enough. But if you increase the size of the pulleys (for a given ratio) the tension in the belt reduces, and hence the scope for slipping is less. So if you can, you would be better advised to get a larger pulley on the engine rather than a smaller alternator pulley.

[Tony Brooks]

If a V belt then use a notched one (looks like teeth on the inside) these are designed to drive on smaller pulleys.

[DHutch]

Twin belts make a big difference, effectively doubling the wrap.

 

Twin belts will double the grip, however doubling the wrap increased the grip by more than a factor of two as you get an effect where by the load pulls the belt into the vee, assuming it is a form of vee-belt.

 

 

As said, you can get down to about 2inch on the sort of size section used on boats, after which it gets still and the belt wont go round it, assuming the belt section and pulley profile match, with an alternator around the 70amp size I expect most setups would be fine without an idler pulley.

 

Emilyanne as a fairly extreme pulley setup, with a 18inch pulley on the propshaft (shaft speed around 180-250 rpm), driving a 3inch pulley on the alternator, giving a ratio of around 1:6 and hence a alternator speed of around 1000-1500. The alternator is internally rewired to star topology to give better charging at slow speeds, and gives out around 20amps at 24v.

 

To add to this we cannot get a conventional belt over the propshaft in the location, so have to use T-link splitable belting, initially having finally got an alternator to charge at the speeds it spins we found the belt slipped, so had to get a new pulley and run two belts.

- The shafts are about 2ft apart, and we are unable to use an idler on the T-link, so the wrap is not great.

Daniel

[nicknorman]

Twin belts will double the grip, however doubling the wrap increased the grip by more than a factor of two as you get an effect where by the load pulls the belt into the vee, assuming it is a form of vee-belt.

 

 

As said, you can get down to about 2inch on the sort of size section used on boats, after which it gets still and the belt wont go round it, assuming the belt section and pulley profile match, with an alternator around the 70amp size I expect most setups would be fine without an idler pulley.

 

Emilyanne as a fairly extreme pulley setup, with a 18inch pulley on the propshaft (shaft speed around 180-250 rpm), driving a 3inch pulley on the alternator, giving a ratio of around 1:6 and hence a alternator speed of around 1000-1500. The alternator is internally rewired to star topology to give better charging at slow speeds, and gives out around 20amps at 24v.

 

To add to this we cannot get a conventional belt over the propshaft in the location, so have to use T-link splitable belting, initially having finally got an alternator to charge at the speeds it spins we found the belt slipped, so had to get a new pulley and run two belts.

- The shafts are about 2ft apart, and we are unable to use an idler on the T-link, so the wrap is not great.

 

 

 

Daniel

All sounds a bit tricky, maybe you need a seperate steam driven generator!

[MtB]

I have a 30" crankshaft pulley and 3" alternator pulley on Aldebaran.

 

Seems to chuck out 8 amps perfectly well!

[dmr]

short answer you will be ok but only just.

 

longer answer....how small is small? The size of the big pulley and pulley centres distance also influence wrap.

Assuming the big pulley is not a huge "mtb" jobbie. and assuming you are using an "A" size belt (not a little "Z") then you will just about be ok. Do get a notched belt as suggested, carry a spare, and check tension (and look for signs of black dust) quite often.

 

Approaching 100amps a double belt system would be needed, but finding a double pulley for the crankshaft might not be cheap or easy.

 

............Dave

[Guest]

Just a thought but if the crankshaft pulley is increased significantly, won't this increase the speed of the water pump significantly, and is that a likely problem?