SPZ belt length: Is "Lw" vs "M" important?

[sploo]

I'm restoring an old table saw, and the original belts could do with replacing. The belts are clearly marked SPZ X 825 Lw, so I bought new belts, marked SPZ - 825 M.

The new belts however are a little longer. If I stretch both old and new out on ~60mm diameter pulleys, the centre distance for the pulleys is a good 1/2" further apart on the new belt. I could have understood the old belts being longer due to stretching.

Is there something important with the Lw vs M marking?

Looking at the range of SPZ belt lengths, I see that the 825 has an internal length of 787mm. Taking 25mm off that (to bring the pulley centres approx 1/2" closer) gives 762mm; which is exactly an SPZ800 belt (762mm inside length). I note that 25mm is also the difference between 800 and 825... so does Lw somehow mean the belt is measured differently (and I need an SPZ 800 belt)?

Note that I can adjust the pulley centres on the machine - but the new belts are just a bit too long, even with the max possible centre distance.

 

[flh801978]

In my day job i replace belts every month on a variety of machinery
And in the last 40 years of doing so I always take the old belt to get a new one and disregard any markings
If the belt was so damaged i'll get 3 ...one under one right and one longer. and use the best fitting...
its a nightmare with different manufactures of belts...even the same numbers on them they will be different lengths by up to an inch

Ian

 

[sploo]

In my day job i replace belts every month on a variety of machinery
And in the last 40 years of doing so I always take the old belt to get a new one and disregard any markings
If the belt was so damaged i'll get 3 ...one under one right and one longer. and use the best fitting...
its a nightmare with different manufactures of belts...even the same numbers on them they will be different lengths by up to an inch

Ian

Thanks - I'm ordering online so unfortunately can't go and compare belts, but I'm going to take some measurements of the old belts and order accordingly. Your comment about different manufacturers belts being an inch out would fit perfectly with what I'm seeing. Of course, I'll go and order some SPZ800 belts and find they're much shorter than the supposedly standard length :wink:

 

[sunnybob]

That appears to be an ordinary V belt. any automotive spares place should be able to match your belt. They have a clever gizmo that recreates two pulleys to get the correct measurement.

 

[SammyQ]

Rotary Bearings in Belfast asked for the distance apart of the two pulleys, followed by the pulley sizes...5 seconds later their pc had my belt spec.
There IS a formula 'out there ' in cyber space and some older 'mechanicing ' books.
Sam

 

[sploo]

I measured the outer circumference of the old and new belts, and got (give or take a few mm):

Old: 827mm
New: 846mm

I found a table that shows the outer circumference of SPZ belts:

SPZ 800: 813mm
SPZ 812: 825mm
SPZ 825: 838mm
SPZ 837: 850mm

It looks then like the old SPZ 825 belts are a bit undersized (827mm actual, vs 838mm claimed), and the new belts are little bit oversized (846mm vs 838mm).

I'll order some SPZ 812 belts from the same source, as I can get away with a shorter belt, but if their SPZ 812 is also a bit oversized then it should still be fine.

Incidentally; I've found some sites that list the inside length as being the same as the SPZ number (e.g. SPZ 800 = 800mm inside length; which would make sense). However, on several other sites an SPZ 800 has a 762mm inside length. I do wonder if the discrepancy comes from how the belts are measured and classed.

EDIT: A bit of extra searching leads me to believe the SPZ number is the pitch length. The inside length/circumference is then approx 37mm shorter, and the outer is the inner length plus 50mm (so 13mm larger than the pitch length). That ties up with the table above.