Basic mechanics (I've forgotten)

[Radian]

Both our automatic garage door openers recently packed up within a month of each other. Pretty remarkable seeing as how I installed them when we built the house 25 years ago!

 

First the good news: just one plastic cog on the chain drive shaft was stripped and repair packs including a whole new shaft are available. Now the bad news: Amazon reviews are largely negative about the replacement plastic cogs saying they don't last very long. Regardless I bought one to try and the first thing I noticed was that my old drive shaft had two chain drive sprockets a the top, one big and one small, whereas the replacement kit has just the big one. 

 

 

I have been using the smaller chain drive sprocket all along so I started thinking about the difference between them. It's rather obvious that the larger sprocket will make the door move faster as one rev will pull round more chain than one rev of the smaller one. But what about the load on the plastic cogs? I can't recall any mechanical education I've ever had that covers this but it seems to me that a fixed amount of power must be transferred to lift the door for a complete cycle. The variable being how long it takes. So if it takes longer to transfer the power from the motor, the stress on the drivetrain at any instant would be lower. Is that right?

 

If it is right then the reason these replacement cogs wear out so fast might be down to the missing small sprocket. For now, I've just removed the pins holding the cog and transferred it onto the original drive shaft so I can refit the chain on the smaller of the two sprockets just like it has been for the last 25 years.

[ProDave]

Assuming the chain is what pulls the door up, using it on the small sprocket the door will lift slower.

 

So being forced to use the large sprocket (because the small one is no longer there) would lift the door further for each rotation, thus to my simple mind the torque on the shaft would be more and therefore forces on the teeth would be more.

 

So I would say the small sprocket is what you want.

 

Will you be around in 25 years to replace it again?  

[Radian]

2 hours ago, ProDave said:

So I would say the small sprocket is what you want.

 

 

Good, that chimes with my instincts. Everyone goes on about how this same mechanism has been in Chamberlain Liftmasters since 1985, yet the double sprocket obviously hasn't been and at some point after the late 90's it was changed to the 'faster' single sprocket. Probably to keep up with the speed of competing brands (and with the bonus side effect of making the product fail sooner after warranty). I'm glad I've still got the option. In fact I didn't need to buy the whole assembly as the plastic cog can be bought on its own. I'll be getting one of those for the other door. At least the door we use most is working again now.

 

2 hours ago, ProDave said:

Will you be around in 25 years to replace it again?  

 

Only in the extremely unlikely event that I live for as long as my Mother has (so far).

[SimonD]

I think you might be better off measuring up the gear and then ordering a replacement spur gear with pin holes from a bearing supplier or other industrial supplier. That way you'll most likely get the right diameter and maybe a better quality gear.

 

You'll need to know the module: http://www.metrication.com/engineering/gears.html

 

 

 

 

[SteamyTea]

Half the ratio and the torque doubles for the same power.

[Radian]

9 hours ago, SimonD said:

I think you might be better off measuring up the gear and then ordering a replacement spur gear with pin holes from a bearing supplier or other industrial supplier. That way you'll most likely get the right diameter and maybe a better quality gear.

 

You'll need to know the module: http://www.metrication.com/engineering/gears.html

 

 

 

 

 

It's not a straight cut gear though. It's driven by a worm gear so I doubt if I'd find any off-the-shelf replacement in metal. Even if I did the worm drive would be the next in line for wear. And on it goes...

[dpmiller]

looking at the wear, I've got to ask a. can you bring the worm down a bit for deeper engagement and b. what kind of grease are you using?

[Radian]

13 minutes ago, dpmiller said:

looking at the wear, I've got to ask a. can you bring the worm down a bit for deeper engagement and b. what kind of grease are you using?

 

That's a good point. I don't think the motor has any lateral adjustment which could help with this. This photo montage shows how the worm drive sits on the motor shaft. The grease came in a tube like the one shown below.

 

[dpmiller]

oversizing the mounting holes on the bracket for the motor would be an option, and slather a load of grease on it, white "plastic" grease or PTFE bicycle grease (very similar)) would be my choice

[Radian]

The little tube that came with the kit had a white grease. I could do with greasing the chain - does bicycle grease cover that application as well?

[dpmiller]

yes, the chain isn't so critical.

 

Presuming the parts interchange and there's no real wear, why not just use the old shaft so you can have the smaller sprocket?

[MikeSharp01]

9 hours ago, Radian said:

does bicycle grease cover that application as well?

Yes - but don't get any of it on the gears or the grease in the pack (unless it is a lithium based one) - the vast majority of plastic gears are not happy when lubricated with petroleum based lubricants. This site knows what it is talking about. 

[markc]

11 minutes ago, MikeSharp01 said:

Yes - but don't get any of it on the gears or the grease in the pack (unless it is a lithium based one) - the vast majority of plastic gears are not happy when lubricated with petroleum based lubricants. This site knows what it is talking about. 

So true, similarly nylon swells with WD40 causes us loads of problems with sliding collars on air and hydraulic cylinders and interlocks when people blather them in WD40

[SteamyTea]

1 hour ago, markc said:

So true, similarly nylon swells with WD40 causes us loads of problems with sliding collars on air and hydraulic cylinders and interlocks when people blather them in WD40

Is that because WD40 displaces the small water molecules in the nylon and the gaps are replaced with the larger molecules of the oil.

[markc]

13 minutes ago, SteamyTea said:

Is that because WD40 displaces the small water molecules in the nylon and the gaps are replaced with the larger molecules of the oil.

Interesting, I have no idea why but it does only occur with softer nylons, cast nylon6 and 66 are not too bad and Delron seems pretty unaffected but these are all engineering materials. The softer general purpose nylon bar and sheet seem to absorb the WD and surface 0.5mm or so swell .

[Radian]

2 hours ago, dpmiller said:

Presuming the parts interchange and there's no real wear, why not just use the old shaft so you can have the smaller sprocket?

 

Yes, that's exactly what I did. The only minor difference was a smaller roll pin holding the gear wheel on the new shaft but a little filing made it fit with the larger pins on the old shaft. Now to do the other door I'll just buy the replacement gear - even cheaper! The amazon reviews for these suggest they last about a week so I'll make sure the worm is fully engaged and by sticking with the small sprocket, fingers crossed, it'll hold up.

 

As with all these kinds of jobs, now I've done it once, I reckon I could get it done inside 15 minutes. It's certainly a lot easier than replacing the entire units and considerably cheaper! Funny thing is 25 years ago when I got them we picked them up from a B&Q clearance sale in Poole when they enlarged the shop to  Depot. Everything was half price or better so we grabbed two door lifts and a bunch of remotes for less than the price of one motor alone.

 

Doesn't beat the bargain I got in Payless DIY Dorchester when it closed around ten years ago - in the lighting section it was marked 10p for every light bulb. So we filled up the trolley with LED's (about £10 each normally at that time) and whistled all the way to the checkout! Pretty much converted the whole house for half the price of a single LED.

[ToughButterCup]

1 hour ago, SteamyTea said:

Is that because WD40 displaces the small water molecules in the nylon and the gaps are replaced with the larger molecules of the oil.

 

Ya larn sum stoof on this board .....  

 

So @SteamyTea, how do the larger (WD40) molecules force their way into a smaller hole previously occupied by the smaller water molecules  then?

 

"Dwahhling, does my molecule look big in this hole?"

"Ermmm, ummmm , just a bit"

"Well, if it does, you shouldn't be lookin' " 

[SteamyTea]

15 minutes ago, ToughButterCup said:

So @SteamyTea, how do the larger (WD40) molecules force their way into a smaller hole previously occupied by the smaller water molecules  then

Not sure.

Could be that one end of the WD (stands for water displacement) is small and attracts the water out of a hole, them wiggles it's way in.

The other end of the molecule may be larger, so, though electrostatic forces gets drawn in. This will act as a wedge, causing the surface to swell.

 

Guessing a fair bit here. I tend to avoid chemists. They have 20 words and just keep rearranging them.

[markc]

And now it sounds like a biology lesson 😉

[SteamyTea]

4 minutes ago, markc said:

And now it sounds like a biology lesson 😉

Biology is just a Latin test.

 

I read a good book about epigenetics. Only thing I learnt was the Dutch went hungry and the reason it is so hard to make new drugs.

Drugs need to be small molecules so they can pass through cell walls. Most of the useful small ones have already been discovered.

[ToughButterCup]

1 hour ago, SteamyTea said:

...

The other end of the molecule may be larger, so, though electrostatic forces gets drawn in. This will act as a wedge, causing the surface to swell.

....

 

And right here is the reason I keep coming back to BuildHub. Thanks very much.