My belt broke - motor's timing pulley isn't secure

[swbluto]

So - how do you determine the torque at zero RPM without doing the magnetic field calcs?

Motors have this thing called a torque constant. It basically tells you what the torque will be if so many amps are flowing through the motor. The motor also has an internal resistance - the resistance of the copper that makes up the coils. The motor will demand so many amps for a given applied voltage due to Ohm's law - current = voltage/resistance. At 0 RPM, you know the voltage and you know the resistance, so you can find the current at 0 rpm. With the current, using the torque constant, you can find the torque.

The torque constant also has a mathematical relationship with Kv, or the motor's voltage RPM constant, so you can determine the torque constant if you know the voltage RPM constant. So, basically, if you know voltage-RPM constant and the motor's resistance, you can determine the start-up torque.

If the battery current is being limited, there's some additional complications, but the same basic idea applies.

Basically, my simulator (below) does all these calculations automatically so I can find out what the heat generated, the current in the motor, and the torque at 0 RPM is without manually doing somewhat complicated, tedious calculations.

 

[Joepostal]

Hey swbluto,

Damn, I like your scooter. I am going to have to build one... since no one chimed in with the reason it is called a timing pulley I thought I would. SO it is a timing pulley because it has teeth. The fact that you can connect a belt between two of them and guarantee that the shaft will be in a certain position after a given number of turns is why.

The force is across the width of the belt (as long as the pulley is a snug fit to the shaft) but if one side of the belt is tighter than the other the force will be unequal towards the tight side.

Did you find a new belt yet. You can probably find something much stronger... Was hoping to see the new one too. I saw some very tough belts mentioned before.

How well is the glue holding the magnets? I was on vacation...

Ok so back to the shear strength of pins
Grabs popcorn.

 

[JS Tyro]

The torque constant also has a mathematical relationship with Kv, or the motor's voltage RPM constant, so you can determine the torque constant if you know the voltage RPM constant. So, basically, if you know voltage-RPM constant and the motor's resistance, you can determine the start-up torque.

So if you don't have the manufacturer's rated torque constant or Kv, you'd have to run a test to measure the Kv by fiddling the V and counting the rpm each time. Calculate the ratios (plural - one data point does not a good measurement make) and see that you get a reasonably consistent value.

Ok, with that, determine the max torque and then the max shear on the pins. Multiply by a number somewhat greater than 2 (2 for dynamic loading and more for the safety factor) to determine the shear capacity required. The smaller the number, the more likely you'll need walking shoes.

 

[Miles]

I'd guess that the belt would ratchet on the motor pulley at around 4 Nm of torque, at most ......

 

[swbluto]

Okay, so I went to drill the hole and apparently the drill bit went all the way through the shaft and the timing pulley, so I thought the holes were aligned. I then went to start hammering the tension pin in and when it came to the other side of the timing pulley, the hole had move by 1/2 the diameter of the 3/32" hole! I tried manually adjusting it but it didn't seem possible with just the hands on the motor. So I'm trying to remove the tension pin by pulling it out and I'm guessing it's not designed to be taken out that way as it seems pretty much impossible. The timing pulley is still secured to the shaft so... any ideas how to take this thing apart???

Why, oh why, does it seem like mechanical things hate me so much? :x

Oh, and just thought I'd say "Stay away from monsterscooters.com". I've had good luck with them in the past, but if you're looking for brand name parts, look away. I tried finding some Gates HTD belts and I saw pictures of some in the description of their web page on a suitable belt and so I purchased it and found out, hah, they ripped the picture off from somewhere else! I got some cheap no-name belts now.

 

[Joepostal]

Awe damn man sorry that the hole wasn't straight. I just purchased a drill press for that exact kind of trouble

Total bummer on the belts as well. Hopefully the EV gods will smile on you tomorrow.

As I live vicariously through your troubles I can't wait to hear about your next successes.
Joe

 

[swbluto]

Ahh, well, I should have probably put something in the other side to keep it from shifting away (Maybe a hex? Maybe a drill bit?). Once I get it pulled out, I think it should be relative smooth sailing from there as I believe I have a "no error" plan in mind now. (And, yes, I used a drill press for this.)

I know if I'm motivated enough, I can get it removed one way or another, but I've found things are a lot easier if you take other people's advice.

 

[Storm]

Did you spin the pulley on the shhaft causing it to not line up u know 180*?

 

[swbluto]

Don't know but I don't think knowing the cause is really going to help me at this point, assuming I'm going to "secure it" next time.

Anyways, I'll think I'll have one more tug session with the vice grips before resorting to a last resort - JB weld the pin to something easier to pull or exert a lot of force on.

 

[12p3phPMDC]

you need a punch with a head that is 5/64". You insert the punch on the other side of
the hole and drift the pin out...

Don't give up man....people that don't make mistakes don't do anything. :wink:

You might have drilled too fast.. Small bits like to flex. Go slow, especially at first, until
a pilot hole gets established. If the beginning is straight it's much more likely the rest of
the hole will be.

Also, the drill press table may be slightly out of square with respect to the chuck center line.
You can chuck up a piece of drill rod and place a square on the table and the drill rod. Adjust
the table until you see no gap between the square and the drill rod.

Did you use any lubricant while drilling? A little motor oil or drill and tap oil lube can really help.
It also shows you when you are going to fast. The bit will heat up and cause the oil to smoke.
That's too hot and will dull the bit as well.

 

[swbluto]

Since the hole isn't aligned, "the other side" isn't really accessible from the tension pin's point of view. So it seems it must be pulled out one way or another.

EDIT: Yay, I pulled out! Thank goodness for vice grips and flathead screw drivers (I notice they can impart a significant amount of leverage)!

Now, back to the hole, gaahhhhh... "fixing the other side" by putting a drill bit in results in the other one being automatically mis-aligned. It seems that the holes on the timing pulley and the shaft aren't aligned even though the drill bit drilled right through both of them at the same time.

Ok, what to do next. It seems I might have to drill another hole through one of the parts...(And the motor's shaft doesn't sound like an appealing option). I think I'll reread the previous post.

 

[swbluto]

Also, the drill press table may be slightly out of square with respect to the chuck center line.
You can chuck up a piece of drill rod and place a square on the table and the drill rod. Adjust
the table until you see no gap between the square and the drill rod.

That seems like it might have been the problem. I don't really do much to ensure alignment, other than suspending the motor in mid air with my hand and trying to make sure the timing pulley was resting flat against the drill's square drilling area. I thought, though, that drilling through both at the same time all the way through would've ensured alignment.

 

[12p3phPMDC]

Hmm....

So, it sounds like the pin was able to go all the way through the pulley the shaft and the pulley?
or just the pulley and the shaft, then wouldn't go through the other side of the pulley?

If the drill bit went all the way through then it seems like the pin should really go all the way through.
Maybe the pulley just shifted a bit when you started tapping it in?

A vice or "Jorgensen" parallel wood clamps would really help you hold things....and it's safer.
Clamping round things is slightly tricky. A good assortment of hole saws can help you approximate
the diameter of things. Also V blocks help you clamp stuff. Insert round thing into V with flat bottom constraing
the piece at two points instead of one.
o
_V_

 

[swbluto]

Ok, thanks for the advice. It does seem a bit unusual to secure round things...

Anyways, I reverted back to the set screw since I wanted to get this thing going as soon as possible (I was going to do the spring pins later when I clearly figured out what I did wrong and I knew what to do). In the process of doing so, I flipped the belt over the pulley and... it wouldn't go on. I eyeballed it and it seemed like the 15mm width was a little too much for it (Like by 2 mm). What??? I don't have the original broken belt on me but I could've swore it said something to that effect, and I wouldn't have just made up a number for my convenience. I eyeballed the next common size, 9mm, and that seemed to be too thin for its original width. I'm starting to think my original belt was actually 12mm wide or something... or maybe they (Viza Motors) chopped off a couple of mm when they receieved their "15mm" belts in bulk to get it to fit.

Gahhhh, it seems like madness. Going to the official page, it seems like they mention all the dimensions BUT the width.. http://www.vizamotors.com/volt-drive-belt-500mm-pitch-original-p-29.html - Does that suggest this is a special company-specific belt?

I measured the new belts and they were indeed 15mm wide, so monster scooters wasn't lying.

(I think I'm starting to learn you need to measure everything where size is critical. Those 4"x4" pieces of lumber are actually 3.5"x3.5" or they were at home depot, anyways.)

 

[swbluto]

Anyways, I directly ordered from viza volt hoping they would at least have the right parts. I don't really have much time to ponder these things as I *must* have an RC motor enabled vehicle for testing purposes. And, as much as I want to use the scooter, I think I'm going to have to restrict it to those purposes as I can't afford break downs (in terms of time). Not in the near future, anyways.

By the ways, where would one find a "vee block"? Do they sell those at lowe's or home depot?

 

[12p3phPMDC]

I fabricate my own by cutting two 45 cuts in a piece of wood with table saw...typically
for drilling holes in round table legs or similar.

You can buy more expensive metal v blocks at MSC, but look out! other places may have cheaper
ones... look for an import supply house, maybe ENCO.

A hand saw should be sufficient for cutting a v block in a small wood block like a 3.5" x 3.5" x 4" long or so from "4x4s".
Shim the motor to the right height as needed.

If you want lumber that is closer to the actual rating, you have to get "rough" cut lumber from
a timber supply where a 4x4 is 4" x 4" +- accuracy of the saw. A 4x4 from the store is called "dimensional"
lumber, which doesn't make much sense either...Maybe because it is cut to "common" dimensions accurately.
It's a bit strange to the uninitiated, much like alot of other things like wire gauge and sheet metal gauge, pipe size,
etc... The origins of which are kind of shrouded in history and god only knows what else. Gauge systems are geometric progressions, but are not perfect ratios either. They are probably rounded for measurability to a thousanth of an inch.

But, your right, it pays to measure just about everything when adapting/retrofitting/building things.

You may want to befriend a machinist or good shop person that can help with tool and machine guidance.
I grew up in a shop with my old man...I'm not the best machinist or woodworker in the world, apprentice level if
I had to classify it. He was a master craftsman. It helps to have guidance. Community colleges have cheap
classes with shop access too! Good luck.

 

[JS Tyro]

For checking the alignment of the drill press table, get a small engineer's square engineer's square http://www.leevalley.com/wood/page.aspx?c=1&p=32601&cat=1,42936 (they are generally available from many places that sell good wood or metal shop equipment). Use it to confirm that the drill bit is at right angles to the table in more than one direction (i.e check it from left to right as well as front and back).

You might consider buying a modestly priced V block http://www.wttool.com/product-exec/product_id/19427?utm_medium=cpc&utm_source=froog. While you could make one with wood, if you don't get your cuts just right, the V will not be parallel to the base and you'll have an alignment problem, defeating the purpose of aligning the drill bit and table.

Note that a sharp drill bit will cut a nice hole while a slightly dull bit will tend to drill a slightly oversized hole - that's because the bit will wobble around as it tries to bite into the metal. A very dull bit will produce heat and not much else. Cutting oil is a very good idea when drilling metal, especially steel. Make sure you are using HSS drill bits for metal and not plain ordinary wood bits. I have a gazillion different types of bits for different materials and holes; almost every new project seems to need a new type of bit

 

[swbluto]

Cool, I'll look into that.

I just found my broken belt and saw that it never mentioned 15mm and it in fact never mentioned the width. I just assumed that, apparently, since it seemed like a common standard for small electric scooters. I measured it and it turns out to be 11mm wide. So, I guess this is a special part? It seems the surrounding two common size standards is 9mm and 15mm.

Does anyone know of a way to cut a 15mm belt down to 11mm?

And I'm looking at a few places for 500ish-5m-11-12mm drive belts and, I'll be darned, a few do exist. *slaps forehead* But, they're apparently not common enough to have any selection in quality. I'm betting that only one or two manufacturers make them.

 

[Miles]

Does anyone know of a way to cut a 15mm belt down to 11mm?

http://www.pfeiferindustries.com/timingbelt_sliptoanywidth.htm They won't cut your existing belt down, though...

 

[recumpence]

I have narrowed belts with a charp packing knife. Just take your time.

Matt

 

[swbluto]

I've narrowed the belt. I also have suitable replacement belts, now.

Does anyone know of a glue that can be used to secure the timing pulley so it doesn't "wobble", like maybe epoxy? But, yet, there's some feasible way to remove the epoxy when I need to remove the timing pulley? I can use "high heat" (Like 400-500 degrees, maybe 600-700 with great effort), if that's a possibility, or possibly some kind of solvent if some exists.

I need to be able to remove the timing pulley to be able to service the motor, which seems like I'll need to be able to do periodically.

 

[dontsendbubbamail]

This is an untried idea. You might consider using the thread repair epoxy that is found in auto parts stores. It comes with a release agent. Use it to build up the part that is causing the wobble. I forget if the pulley or the shaft is the cause of your wobble. Let's assume the pulley is good. Put the release agent on the pulley, spread epoxy on the shaft, slide the pulley on, hold the pulley centered on the shaft until the epoxy dries. It is fast cure epoxy. Now you can slide the pulley off the shaft without having to heat it. The epoxy most likely has metal in it, so you may want to remove the shaft from the bell while you try this.

Bubba

 

[swbluto]

This is an untried idea. You might consider using the thread repair epoxy that is found in auto parts stores. It comes with a release agent. Use it to build up the part that is causing the wobble. I forget if the pulley or the shaft is the cause of your wobble. Let's assume the pulley is good. Put the release agent on the pulley, spread epoxy on the shaft, slide the pulley on, hold the pulley centered on the shaft until the epoxy dries. It is fast cure epoxy. Now you can slide the pulley off the shaft without having to heat it. The epoxy most likely has metal in it, so you may want to remove the shaft from the bell while you try this.

Bubba

I will try it! That sounds like it might be what I'm looking for.

It's the pulley whose bore is slightly oval, but hey, it could be also said the shaft just isn't oval enough. Would the procedure be reversed if the timing pulley was at fault? Epoxy in the timing pulley and release agent on the shaft?

 

[swbluto]

so you may want to remove the shaft from the bell while you try this.

Bubba

Oh wait, just read this more carefully. Do you mean take the two halves of the motor apart (the stator with the coils and the bell with the magnets) or remove the shaft from the bell which it appears press fitted into?

The former I've done but the latter I'm wary of doing. Don't know if I could get it back in!

 

[swbluto]

I went to the automotive store and saw that they only had one thread repair kit. It would've cost $15! I know that sounds like pennies to some, but all the little costs seem to be making this one expensive little scooter. So I started looking at the glues available...

And then I noticed one said "Steel" followed by the word "Drillable!". That got me thinking.

What if this compound could fill the entire pulley's bore and then the shaft diameter could be drilled out?

I wouldn't be able to use loctite to secure the pulley, but I was planning on using two set screws for securing it anyways.

I think I'll experiment with this idea using some JB Weld I have and then "bake it" during cure to increase the strength. (I'll probably put it at 200 degrees fahrenheit)

It appears that 3/8" will miss the shaft diameter by about .1 to .2 mm, so I'll just need to either slightly enlarge the hole (if necessary) by some means (Maybe I'll just be more careful with filing, haha) or reduce the shaft diameter by sanding. I think reducing the shaft diameter to an imperial unit might be easier to get parts for it in the US.

I have my doubts this will work as well as I want it to as the sandability of JB weld seems pretty poor (It just chunks off in little chunks, as far as my last experience went), but it uses equipment I already have so the cost should be minimal and I hope that baking it as it cures should improve its drilling and sanding characteristics.