Changing pulley/sheve size

143 · January 29th, 2009, 10:25 AM

Does anyone know if it's ok to replace both the drive pulley and the blower pulley/sheave with smaller ones while keeping their ratios the same? For example if I have a 10" blower sheave and a 6" drive sheave, can I go with a 5" and a 3" combinaton so that I do not tamper with the cfm/rpm. Will this impact my equipment longivity or perfomance.

TheSuper · January 29th, 2009, 03:19 PM

Very intelligent question 143. Here are my assumptions:

A) As you very smartly state stepping down the ratio propotionately, I would guess it should not affect the cfm/rpm, therefore it will remain the same.

b) However, the motor will have to work very much harder (needs more torque) on start up to get the fan turning. Therefore, I would say the longevity of the motor will be compromised. I'm no expert on this and this is just a guess. The motor rpm will remain the same but the fan rpm will drop due to the gearing down effect, I would guess thereby reducing fan speed and CFM.

Under the above conditions can the starting torque be stepped up by upgrading the start capacitor without altering or frying the motor??

143 · January 29th, 2009, 04:59 PM

Super........now you are getting me confused

. Yoy mentioned in a) that the rpm/cfm will not change while in b) you're saying that it will change ??? Also what are the reasons supporting your saying that the motor will/might work harder and that the motor will require more torque.

TheSuper · January 29th, 2009, 06:02 PM

Quote:

Originally Posted by 143

Super........now you are getting me confused

. Yoy mentioned in a) that the rpm/cfm will not change while in b) you're saying that it will change ??? Also what are the reasons supporting your saying that the motor will/might work harder and that the motor will require more torque.

They were assumptions. First I typed (A) and it did and then didn't make sense. So I typed (B).

Now that I'm in a better frame of mind, let me go with B and let me elobrate how my minds sees it.

If you gear down the sheave as you suggest (reducing size in half), the fan rpms drop by half and so should the CFM. On start up, a bigger sheave requires less torque to get going vs a smaller sheave exerting more stress on the motor hence my theory the motor will have a diminished service life.

143 · January 30th, 2009, 12:22 AM

Super........pay close attention when I said that the pulley ratio stays the same. The cfm of a fan is determined by the size/type of blower and the rpm of the fan which is governed by the rpm of the motor. Assumming the original size of the motor pulley to be 6" and the fan pulley 10"
then
circumference of motor pulley is 3.14 x 6 = 18.84"
circumference of blower pulley is 3.14 x 10 = 31.4"

therefore for every 1 rev of motor pulley the blower pulley will make a 0.6 rev.

Now let's substtute our new figures.......5" and 3"

circumference of motor pulley is 3.14 x 3 = 9.42"
circumference of blower pulley is 3.14 x 5 = 15.7

again for every one rev of motor the blower will rotate 0.6 rev.

Bottom line.........the rpm and cfm stays unchanged. As a matter of fact if the amperage of the motor is taken during our second example it will tend to read slightly lower...............confused?????

TheSuper · January 30th, 2009, 02:12 AM

Quote:

Originally Posted by 143

Bottom line.........the rpm and cfm stays unchanged. As a matter of fact if the amperage of the motor is taken during our second example it will tend to read slightly lower...............confused?????

Excellent explanation 143 and I'm not confused. I see "clearly" now

. I'm now trying to understand how and why the amp reading would be lower in this case. Smaller pulley's, less weight?

+Rep coming you way.

walkerguy · February 1st, 2009, 10:11 AM

You are right 143. If the ratio between the motor pulley and blower pulley is unchanged the rpm and the cfm will not change. The relation between rpm and the dia is as follows:
RPMmotor /Dia blower = RPMBlower/DIA motor
As far as change in Amp I am little bit confused because if there is no change in the rpm (unless the motor is changed to a higher rpm) the Amp should not be changed According to the Amp calculation it is also verified
New Amp= existing Amp x {new rpm/existing rpm} ³
Theoretical Amp should not changed if the rpm is not changed but if we consider the weight of the pulleys there could be slight variation in Amp but not significant.

143 · February 1st, 2009, 09:58 PM

You're absolutley right Walkerguy by saying that the amp variation is only going to be slight. I was trying to debate in respond to The Super's answer which he guessed that the amp will increase. Let me justify my explaination.
As you know, a lot of physics is taken into consideration when selecting a belt/pulley combination mainly torque,power, belt wear & tear, heat rejection caused by friction etc. The ability of a drive motor to move a fan efficiently depends on the the motor hp and the torque applied to the fan pulley which is directly proportional to the belt to pulley contact area.
By reducing the size of the two pulleys, the contact area of the pulleys is reduced thereby reducing grip and increasing slippage during start-up. This will compromise belt/pulley longivity. As for the slightly lower amperege, that is due to the drop in load on the motor due to slippage.

January 29th, 2009, 10:25 AM	#1
143 Member Join Date: Jan 2009 Posts: 43	Changing pulley/sheve size Does anyone know if it's ok to replace both the drive pulley and the blower pulley/sheave with smaller ones while keeping their ratios the same? For example if I have a 10" blower sheave and a 6" drive sheave, can I go with a 5" and a 3" combinaton so that I do not tamper with the cfm/rpm. Will this impact my equipment longivity or perfomance.

January 29th, 2009, 04:59 PM	#3
143 Member Join Date: Jan 2009 Posts: 43	Super........now you are getting me confused. Yoy mentioned in a) that the rpm/cfm will not change while in b) you're saying that it will change ??? Also what are the reasons supporting your saying that the motor will/might work harder and that the motor will require more torque.

January 29th, 2009, 03:19 PM	#2
TheSuper Administrator Join Date: Jan 2009 Location: Plano, Texas, Posts: 99	Very intelligent question 143. Here are my assumptions: A) As you very smartly state stepping down the ratio propotionately, I would guess it should not affect the cfm/rpm, therefore it will remain the same. b) However, the motor will have to work very much harder (needs more torque) on start up to get the fan turning. Therefore, I would say the longevity of the motor will be compromised. I'm no expert on this and this is just a guess. The motor rpm will remain the same but the fan rpm will drop due to the gearing down effect, I would guess thereby reducing fan speed and CFM. Under the above conditions can the starting torque be stepped up by upgrading the start capacitor without altering or frying the motor??

January 30th, 2009, 12:22 AM	#5
143 Member Join Date: Jan 2009 Posts: 43	Super........pay close attention when I said that the pulley ratio stays the same. The cfm of a fan is determined by the size/type of blower and the rpm of the fan which is governed by the rpm of the motor. Assumming the original size of the motor pulley to be 6" and the fan pulley 10" then circumference of motor pulley is 3.14 x 6 = 18.84" circumference of blower pulley is 3.14 x 10 = 31.4" therefore for every 1 rev of motor pulley the blower pulley will make a 0.6 rev. Now let's substtute our new figures.......5" and 3" circumference of motor pulley is 3.14 x 3 = 9.42" circumference of blower pulley is 3.14 x 5 = 15.7 again for every one rev of motor the blower will rotate 0.6 rev. Bottom line.........the rpm and cfm stays unchanged. As a matter of fact if the amperage of the motor is taken during our second example it will tend to read slightly lower...............confused?????

February 1st, 2009, 10:11 AM	#7
walkerguy Junior Member Join Date: Jan 2009 Location: Markham Gender: male Posts: 8	You are right 143. If the ratio between the motor pulley and blower pulley is unchanged the rpm and the cfm will not change. The relation between rpm and the dia is as follows: RPMmotor /Dia blower = RPMBlower/DIA motor As far as change in Amp I am little bit confused because if there is no change in the rpm (unless the motor is changed to a higher rpm) the Amp should not be changed According to the Amp calculation it is also verified New Amp= existing Amp x {new rpm/existing rpm} ³ Theoretical Amp should not changed if the rpm is not changed but if we consider the weight of the pulleys there could be slight variation in Amp but not significant.

February 1st, 2009, 09:58 PM	#8
143 Member Join Date: Jan 2009 Posts: 43	You're absolutley right Walkerguy by saying that the amp variation is only going to be slight. I was trying to debate in respond to The Super's answer which he guessed that the amp will increase. Let me justify my explaination. As you know, a lot of physics is taken into consideration when selecting a belt/pulley combination mainly torque,power, belt wear & tear, heat rejection caused by friction etc. The ability of a drive motor to move a fan efficiently depends on the the motor hp and the torque applied to the fan pulley which is directly proportional to the belt to pulley contact area. By reducing the size of the two pulleys, the contact area of the pulleys is reduced thereby reducing grip and increasing slippage during start-up. This will compromise belt/pulley longivity. As for the slightly lower amperege, that is due to the drop in load on the motor due to slippage.

Changing pulley/sheve size

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