BX HST FAN REPLACEMENT: The easiest way

[Blue280z]

A colleague says that he writes software because he is lazy and wants to do less work. The thought was contagious so here is the lazy (easy) way to replace the notorious broken HST fan on a BX25.

There are many documented ways to do this: Remove deck and drive shaft, cut holes and remove drive shaft, remove drive shaft then split and rivet, scab on split fan over remnants with self-tap tools, hoist engine and move forward, use a 3d printed click together fan (cool!).... and perhaps more I missed. Here is a new way:


0: Back machine up onto car ramps to get easier access and remove bottom shield/plate.

1. Cut and peel off old fan using utility knife and channel lock pliers. (Trick: you just need to expose the two 12mm head bolts that hold the fan to the universal joint)

2. Use a 12mm ratcheting box end wrench to remove the two bolts. (Trick: rotating drive shaft so that the bolt is at the lowest position helps)

3. Cut/peel away old fan.

4. Cut a single slit in new fan hub ( from center rectangle opening to the mid point of the two blades midway between the two round bolt holes... DO NOT CUT FAN IN HALF)

5. INSTALL THE FAN BACKWARDS!!!! <<<< THIS IS THE TRICK TO AN EASY LIFE!!!! The blades rotate in the same direction, this trick allows the 2 bolts to be exposed for installation. The net effect is to simply move the fan ~ 1.5" further from the HST... not a big sacrifice. With the slit in the fan, it is easy to twist over the thin splined HST shaft BETWEEN THE HST BODY AND DRIVE SHAFT UNIVERSAL.

5. Rotate the drive shaft so that the bolt hole is in he lowest position. With your head under the HST and feet out the left side of the tractor, use your left fingers between the gaps in the cross tubes to apply pressure to push the bolt axially into place after using your right hand to pick up the bolt and place it approximately in place. THIS IS THE TOUGH PART... someone with small long fingers would be best. Once you get a few threads to grab, use the ratcheting box end to drive the first bolt in only half way. Rotate the shaft 180 degrees to bring the second bolt hole to bottom and repeat. Then tighten both bolts home.

 

[Russell King]

If the fan is mounted backwards and the shaft rotates the same direction, the fan will be blowing air away from the HST instead of towards the HST.

That may be better than no fan but it is NOT cooling the HST as much as it should be.

Please make sure I am correct and post back what you can see about the air flow.

Can you mount the fan in the correct direction and install the bolts using a socket and extension?

Another possibility is to install studs into the bolt holes while the fan is off, then install the fan over the studs and install nuts to hold the fan on.

Edit - struck through incorrect statement.

 

[GSD-Keegan]

I’m thinking the integrity of the fan will be compromised once it’s cut .

 

[WI_Hedgehog]

If the fan is mounted backwards and the shaft rotates the same direction, the fan will be blowing air away from the HST instead of towards the HST.

Please make sure I am correct and post back what you can see about the air flow.

@Blue280z is correct, the blades blow the correct direction. Try drawing a diagonal line on a sheet of paper and you'll see this to be the case.

GENERALLY fans have a curve to the blade to scoop air, IF that's the case with this fan (I can't go look for the time being) there will be a significant loss of pressure as the air would then roll off the blade; coupled with the added distance the configuration isn't ideal, but would at minimum serve as a short-term solution.

Quite creative @Blue280z, thank you for sharing!

 

[GreensvilleJay]

Wonder how long it'd take to pay off a 3D printer ? Still like my idea of a 2 identical piece unit.
Really... how many owners would pay $50 for the improved fan ?

 

[Russell King]

@Blue280z is correct, the blades blow the correct direction. Try drawing a diagonal line on a sheet of paper and you'll see this to be the case.

GENERALLY fans have a curve to the blade to scoop air, IF that's the case with this fan (I can't go look for the time being) there will be a significant loss of pressure as the air would then roll off the blade; coupled with the added distance the configuration isn't ideal, but would at minimum serve as a short-term solution.

Quite creative @Blue280z, thank you for sharing!

Yes I see that so will change the post above. I was thinking incorrectly about the rotation direction. It may not be as efficient but the flow will be the same direction.

 

[Henro]

@Blue280z is correct, the blades blow the correct direction. Try drawing a diagonal line on a sheet of paper and you'll see this to be the case.

GENERALLY fans have a curve to the blade to scoop air, IF that's the case with this fan (I can't go look for the time being) there will be a significant loss of pressure as the air would then roll off the blade; coupled with the added distance the configuration isn't ideal, but would at minimum serve as a short-term solution.

Quite creative @Blue280z, thank you for sharing!

Are you sure about this? For an axial fan?

If this were true, that would mean the fan would only blow in One Direction regardless, which way it turned. I believe some ceiling fans can reverse direction and they blow down in One Direction and they blow up in the other direction.

Common sense says when you reverse an axial fan, even if it has straight blades without a curve, that the angle of the blade will change and if the shaft rotates the same way, the air will flow in the opposite direction.

let’s say the fan was in place and it’s normal orientation as it came from the factory. And that the shaft that the fan was mounted on was made to rotate in the opposite direction from normal. Why would the airflow not change direction?

Granted, if the blades are curved, the fan would be more efficient rotating in One Direction than it would in the other direction. I believe a fan with curved blades might be called a uni-directional fan because it’s much more efficient pushing air in One Direction rather than the other. NOT because it keeps blowing in the same direction when rotation is reversed.

How about posting two drawings to illustrate the validity of your claim?

My goal is understanding what I might be missing…

 

[WI_Hedgehog]

If this were true, that would mean the fan would only blow in One Direction regardless, which way it turned. I believe some ceiling fans can reverse direction and they blow down in One Direction and they blow up in the other direction.

How about posting two drawings to illustrate the validity of your claim?

My goal is understanding what I might be missing…

I understand your chain of thought and had to think about it myself after reading the airflow might be backwards, I think it's a bit of a brain teaser and appreciate your question.

A ceiling fan reverses shaft direction in order to reverse airflow. With the tractor in this instance the fan shaft continues to rotate the same direction, however the fan is installed backwards.

In the following sketch the box on the paper is the fan, the diagonal lines are the blades, and the coaster is the tractor transmission with a shaft coming out of it (imagine the shaft is there).


@Blue280z is putting the fan on backwards, so let's turn our fan sketch the opposite direction like @Blue280z is doing:


The shaft coming out of the transmission continues to rotate the same way, and the fan blades are oriented the same way, so the air flows the same way even though the fan is on "the wrong way" or "backwards."

Not to confuse things, but if the fan were an electric fan then the fan would continue blowing air one way "front to back in relation to the fan" because the motor driving the fan would also be turned around, so in the case of an electric fan which contains its own drive system the air would indeed blow the wrong way. In the case where @Blue280z is only turning the fan blades around and the drive system (the transmission and fan shaft) remains stationary, the air blows toward the transmission.

---
Veering somewhat off-topic, would people in hot climates who work the hydraulic system heavily benefit by mounting a 6" high-flow electric fan in place of the OEM shaft-driven fan? Would people in cold climates also benefit from an electric fan with remote temperature sensor on the hydraulic unit so on startup in cold weather the fan does not cool the hydraulic system and the system can warm up faster? Doing so does add several potential points of failure.

 

[Soopitup]

Are you sure about this? For an axial fan?

If this were true, that would mean the fan would only blow in One Direction regardless, which way it turned. I believe some ceiling fans can reverse direction and they blow down in One Direction and they blow up in the other direction.

Common sense says when you reverse an axial fan, even if it has straight blades without a curve, that the angle of the blade will change and if the shaft rotates the same way, the air will flow in the opposite direction.

let’s say the fan was in place and it’s normal orientation as it came from the factory. And that the shaft that the fan was mounted on was made to rotate in the opposite direction from normal. Why would the airflow not change direction?

Granted, if the blades are curved, the fan would be more efficient rotating in One Direction than it would in the other direction. I believe a fan with curved blades might be called a uni-directional fan because it’s much more efficient pushing air in One Direction rather than the other. NOT because it keeps blowing in the same direction when rotation is reversed.

How about posting two drawings to illustrate the validity of your claim?

My goal is understanding what I might be missing…

I think you have it backwards.

If the fan blades are completely symmetrical they will push/pull the same in either direction.
My ceiling fans are bi directional and the blades are symmetrical.

I know if blades are NOT symmetrical you're not supposed to change the rotation direction because it will effect airflow.
For example radiator fans.

Most fan blades are designed asymmetrically to improve performance.
I don't know if the BX ones are. Not crawling under there today.

 

[GreensvilleJay]

hay , you don't have to crawl under....
remove ROPS,
remove seat,
remove 8-9 levers,
remove fender tin...

TADA, Now you can see the fan from above !!

Just don't have a hernia trying to get the fender tin back on, by yourself ,while lining up 8 or 9 levers....

oh yeah, that PTI spring CAN be put back on,if you remove the left rear tire/rim....

 

[Henro]

I understand your chain of thought and had to think about it myself after reading the airflow might be backwards, I think it's a bit of a brain teaser and appreciate your question.

I was having trouble posting from my phone and had to delete most of your quoted post, but it easily is found a couple posts above.

I made my own drawing so that I can confirm in my own mind what’s going on, and I see what you’re saying now and agree

If the fan itself is turned 180° and the shaft rotation remains the same then airflow direction remains the same. If only the shaft rotation is reversed, airflow will be reversed.

Great talking through this with you!

 

[Henro]

I think you have it backwards.

If the fan blades are completely symmetrical they will push/pull the same in either direction.
My ceiling fans are bi directional and the blades are symmetrical.

I know if blades are NOT symmetrical you're not supposed to change the rotation direction because it will effect airflow.
For example radiator fans.

Most fan blades are designed asymmetrically to improve performance.
I don't know if the BX ones are. Not crawling under there today.

You are correct. Took me a while to see it.

Edit: I’m not sure, but I think the blades on the BX HST cooling fan are twisted a bit so that may define the fan as directional to some degree, So changing its orientation might decrease its efficiency a bit.

 

[Russell King]

I apologize for creating the confusion and admit I was 100% wrong.

I use axial fans all the time at both ends of a motor to drive air into the motor so I should have realized that it is the direction of rotation not the mounting direction that changed the air flow.

I was thinking about how if we want the motor to reverse rotation direction we move the fans end to end. But those fans are built with some blade overlap and simply don’t work well if they rotate backwards.

The fan shown has no blade overlap and will move air the same direction no matter if it is correctly mounted or turned backwards IF the direction of rotation is the same. It will not be moving the same exact amount of air due to other blade features but there won’t be any huge difference in volume and ZERO difference in direction of the flow.

I should have thought about it more carefully before posting my original incorrect post.

I am glad that @WI_Hedgehog pointed out the error and corrected my mistake.

 

[WI_Hedgehog]

From an Engineering standpoint, looking at an idea critically helps ensure it's valid and evaluates its usefulness; that adds value to most any solution. Thanks for adding value!

 

[Blue280z]

i hope this saves a lot of people grief. The reversed fan is not perfect but the trade off is worth it. Lots of air flows!

 

[Henro]

i hope this saves a lot of people grief. The reversed fan is not perfect but the trade off is worth it. Lots of air flows!

Any chance you have a picture of the Reversed fan in place. That would be a great addition to this thread!

 

[Russell King]

And a picture of the 280Z also please!