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Default How to measure pulley diameter

I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro


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Default How to measure pulley diameter

On Thu, 30 Aug 2007 21:31:20 -0500, "MG" wrote:

I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?


You're on the right track. The pitch diameter of the pulleys is
measured at the neutral axis of the belt. If you imagine the belt
being made up of many thin layered loops, the loop that doesn't change
length as the belt is flexed is the neutral axis. The reinforcing
cords in a v-belt define the neutral axis, which is perhaps 1/3 of the
belt thickness from the top surface of a typical v-belt.

--
Ned Simmons
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Default How to measure pulley diameter

On Aug 30, 10:31 pm, "MG" wrote:
I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro


I do see your problem. You allowed 1/2" for the center of the belt on
the 2.5" pulley, and then neglected to do so for the 5" pulley. Why
is that? By your logic, the 5" pulley would actually have an
effective diameter of 4.5", wouldn't it?

And then you should probably allow for the exact spot the belt rides
on the pulleys - it may be a bit over the top of the groove on one and
slightly below on another. And, as Ned mentioned, you should make
your calculations based on about 1/3 the thickness of the belt from
the outside surface.

On the other hand, does it really matter if your saw blade turns at
exactly 3600 RPM? Or at the 3450 RPM that your original motor most
likely runs at? I don't think there will be any trouble if your blade
turns fast or slow by 10 to 15% or so. I wouldn't worry about it, but
I could be wrong.

John Martin

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Default How to measure pulley diameter

John Martin wrote:
On Aug 30, 10:31 pm, "MG" wrote:

I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro



I do see your problem. You allowed 1/2" for the center of the belt on
the 2.5" pulley, and then neglected to do so for the 5" pulley. Why
is that? By your logic, the 5" pulley would actually have an
effective diameter of 4.5", wouldn't it?

And then you should probably allow for the exact spot the belt rides
on the pulleys - it may be a bit over the top of the groove on one and
slightly below on another. And, as Ned mentioned, you should make
your calculations based on about 1/3 the thickness of the belt from
the outside surface.

On the other hand, does it really matter if your saw blade turns at
exactly 3600 RPM? Or at the 3450 RPM that your original motor most
likely runs at? I don't think there will be any trouble if your blade
turns fast or slow by 10 to 15% or so. I wouldn't worry about it, but
I could be wrong.

John Martin


Won't the blade lose power with the pulley reduction? I know it will
run faster than the motor but won't it lose torque? I know a small
pulley driving a big one will gain power to move objects so it stands to
reason that reversing that will give more RPM but less power. Am I wrong
on that?

Jim Chandler
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Default How to measure pulley diameter

On Fri, 31 Aug 2007 03:58:26 GMT, Jim Chandler wrote:

John Martin wrote:
On Aug 30, 10:31 pm, "MG" wrote:

I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro



I do see your problem. You allowed 1/2" for the center of the belt on
the 2.5" pulley, and then neglected to do so for the 5" pulley. Why
is that? By your logic, the 5" pulley would actually have an
effective diameter of 4.5", wouldn't it?

And then you should probably allow for the exact spot the belt rides
on the pulleys - it may be a bit over the top of the groove on one and
slightly below on another. And, as Ned mentioned, you should make
your calculations based on about 1/3 the thickness of the belt from
the outside surface.

On the other hand, does it really matter if your saw blade turns at
exactly 3600 RPM? Or at the 3450 RPM that your original motor most
likely runs at? I don't think there will be any trouble if your blade
turns fast or slow by 10 to 15% or so. I wouldn't worry about it, but
I could be wrong.

John Martin


Won't the blade lose power with the pulley reduction? I know it will
run faster than the motor but won't it lose torque? I know a small
pulley driving a big one will gain power to move objects so it stands to
reason that reversing that will give more RPM but less power. Am I wrong
on that?

Jim Chandler



If the motor is the same HP, it will produce twice the torque at half
the speed. The "gearing" will bring it back to the same speed and
torque.

As for the pulleys, pretty close just measuring either the root or the
crest of the pulley on both - and if they are the same width the speed
ratio will be the same as the measurement ratio. (within a VERY small
percentage of error)

--
Posted via a free Usenet account from http://www.teranews.com



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Default How to measure pulley diameter

On Thu, 30 Aug 2007 20:29:22 -0700, John Martin
wrote:

On Aug 30, 10:31 pm, "MG" wrote:
I have a table saw with motor and arbor pulleys 2.5" max dia.

Motor is 3600 rpm ratio 1:1

I need to replace the motor with a 1800 rpm
Ratio 2:1 to keep same blade rpm.

Easy enough, place a 5" pulley on the motor.

But the belt is about 0.5" thick radially, assuming it make contact in the
middle it reduces the effective pulley diameter to 2" so I may need a 4"
inch pulley on the motor to achieve the correct 2:1 ratio.

Can you see my problem?

Where is the actual motion transfer taking place?

Thanks

Mauro


I do see your problem. You allowed 1/2" for the center of the belt on
the 2.5" pulley, and then neglected to do so for the 5" pulley. Why
is that? By your logic, the 5" pulley would actually have an
effective diameter of 4.5", wouldn't it?

And then you should probably allow for the exact spot the belt rides
on the pulleys - it may be a bit over the top of the groove on one and
slightly below on another. And, as Ned mentioned, you should make
your calculations based on about 1/3 the thickness of the belt from
the outside surface.

On the other hand, does it really matter if your saw blade turns at
exactly 3600 RPM? Or at the 3450 RPM that your original motor most
likely runs at? I don't think there will be any trouble if your blade
turns fast or slow by 10 to 15% or so. I wouldn't worry about it, but
I could be wrong.

John Martin

According to Machinery's Handbook (13 ed. P.877-8)
"These pitch diameters equal approximately the outside diameter of the
sheave minus an amount equal approximately to the belt thickness plus
from 1/16 to 1/8 inch, depending upon the belt size. For example, if
the belt is size E or 1 1/2 by 1 inch, the pitch diameter would equal
( approximately at least) the outside sheave diameter minus 1 1/8
inch."

Approximately approximation of an approximate answer!
This was the first thing I learned after I bought this book for five
bucks ten years ago, something that I had pondered for many years.
Gerry :-)}
London, Canada
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Default How to measure pulley diameter

To measure the blade's pulley "pitch diameter": make a chalk mark on
the pulley and belt just where the belt "exits" the pulley, rotate the
pulley one revolution so the belt's chalk mark moves away from the
pulley, and measure the distance on the belt from its chalk mark to the
pulley's chalk mark. This is the pitch circumference, divide by pi for
the pitch diameter.

Buy a new pulley whose pitch diameter is twice this. Pulleys *are* sold
by pitch diameter. Maybe not the ones on the rack at Ace Hardware, but
the ones at MSC, McM-C, etc.

HTH,
Bob
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Default How to measure pulley diameter

Bob Engelhardt wrote:

To measure the blade's pulley "pitch diameter": make a chalk mark on
the pulley and belt just where the belt "exits" the pulley, rotate the
pulley one revolution so the belt's chalk mark moves away from the
pulley, and measure the distance on the belt from its chalk mark to the
pulley's chalk mark. This is the pitch circumference, divide by pi for
the pitch diameter.

Buy a new pulley whose pitch diameter is twice this. Pulleys *are* sold
by pitch diameter. Maybe not the ones on the rack at Ace Hardware, but
the ones at MSC, McM-C, etc.


Bob, that is a very clever trick, and one I never thought of when I had
this problem!

I actually disagree with you about pulleys being sold by PD, though. If I
go into my local Ace and measure the OD of what they sell as a 5" pulley,
it will read 5" and it will be made by Chicago Die Casting. If I order a
3/4" bore 5" pulley from MSC it will also be made by Chicago Die Casting
and it will also measure 5" across the OD. And it may well have 1/32" of
runout both radially and axially. I've bought and returned pulleys from
MSC.

Maybe if you step up to Browning, Morse, don't know.

It is possible to take such a pulley and mount it by its bore in a lathe
and true it up. I did this when I replaced a Shopsmith drive pulley awhile
ago and the trued up el cheapo zinc pulley runs dead true.

Grant
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Default How to measure pulley diameter

Grant Erwin wrote:

... If I order a 3/4" bore 5" pulley from MSC it will also be made by Chicago Die Casting
and it will also measure 5" across the OD. ...


OK, it seems to be a mixed bag at McM-C: it asks you for the OD when
searching, but the catalog page does show the pitch diameter. BTW - for
3L belts the pitch diameter is 3/32" less than the OD. For 4L, A, & AX
belts it is 1/4" less & for 5L, B, & BX belts it is 1/2" less.

Bob
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Default How to measure pulley diameter

Bob Engelhardt wrote:

Grant Erwin wrote:

... If I order a 3/4" bore 5" pulley from MSC it will also be made by
Chicago Die Casting
and it will also measure 5" across the OD. ...



OK, it seems to be a mixed bag at McM-C: it asks you for the OD when
searching, but the catalog page does show the pitch diameter. BTW - for
3L belts the pitch diameter is 3/32" less than the OD. For 4L, A, & AX
belts it is 1/4" less & for 5L, B, & BX belts it is 1/2" less.

Bob


Is that measured data? If not, what is your source?

GWE


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Default How to measure pulley diameter

Thank you Bob, very useful answer.
For some mysterious reason neither my OP nor your response appear on my news
reader.

Mauro



"Grant Erwin" wrote in message
...
Bob Engelhardt wrote:

To measure the blade's pulley "pitch diameter": make a chalk mark on the
pulley and belt just where the belt "exits" the pulley, rotate the pulley
one revolution so the belt's chalk mark moves away from the pulley, and
measure the distance on the belt from its chalk mark to the pulley's
chalk mark. This is the pitch circumference, divide by pi for the pitch
diameter.

Buy a new pulley whose pitch diameter is twice this. Pulleys *are* sold
by pitch diameter. Maybe not the ones on the rack at Ace Hardware, but
the ones at MSC, McM-C, etc.


Bob, that is a very clever trick, and one I never thought of when I had
this problem!

I actually disagree with you about pulleys being sold by PD, though. If I
go into my local Ace and measure the OD of what they sell as a 5" pulley,
it will read 5" and it will be made by Chicago Die Casting. If I order a
3/4" bore 5" pulley from MSC it will also be made by Chicago Die Casting
and it will also measure 5" across the OD. And it may well have 1/32" of
runout both radially and axially. I've bought and returned pulleys from
MSC.

Maybe if you step up to Browning, Morse, don't know.

It is possible to take such a pulley and mount it by its bore in a lathe
and true it up. I did this when I replaced a Shopsmith drive pulley awhile
ago and the trued up el cheapo zinc pulley runs dead true.

Grant



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Default How to measure pulley diameter

Grant Erwin wrote:
Is that measured data? If not, what is your source?


Those were the differences noted in the McM-C catalog, p 1001. Bob
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