Chain Drives

[The Medway Handyman]

My 7 year old granddaughter (the most beautiful, intelligent & talented child on the planet, in my completely unbiased opinion) has an electric scooter, her pride & joy.

'Gramps' (as I am known) was called upon to repair it. According to the destructions, you are supposed to 'scoot' to get the thing moving, then twist the control to kick in the motor. I guess the motor doesn't have enough welly to start from a standstill.

The scooter had become very hard to 'scoot'. It was almost impossible to get the driven rear wheel to move at all.

So, removed about 89 pesky screws to get the covers off and found a small motor driving the rear wheel via a smaller version of a bike chain. Small cog on motor, big cog on wheel.

The chain was under so much tension it resembled a solid steel bar - no give or slack whatsoever - I mean none.

There were a couple of tension adjusters which I slackened off to give a little play, then I liberally applied some lithium grease.

Bloody thing now goes like a rocket - and I am now referred to as 'Super Gramps' :-)

Made me wonder though what the rules of play are regarding chain drives? Is some slack essential to get maximum power transmission? Or should the chain be under maximum tension? Maybe I should have left the adjustment & just applied the grease?

[Matty F]

As long as the chain doesn't slip, it would be best to leave it slack. The force on the bearings from a tight chain is bad for them. The chain on my motorbike was too tight and the gearbox bearing collapsed and strew metal chips throughout the gearbox and wrecked every gear.

[newshound]

You did just the right thing.

The power is being transmitted by the "tension" side of the chain, and it gets its tension from the act of driving; unlike a belt, you don't need any friction because you have the teeth. It's OK for the non drive side to be slack, just so long as there's not so much that the chain can fall off, or that "backlash" causes a problem. If there is excess tension in the chain you are just loading up the bearings at the input and output end. Also, because no chain has perfectly constant pitch, if you are just tight in the slackest part then you have tension the rest of the time. You want to aim for "just slack" in the tightest part.

Does it have "suspension" like a motorbike? If so, the same logic applies, but you need to set the tension with the suspension in the tightest position.

Lithium grease is (sort of) OK, but you want moly disulphide to get the lowest friction (hence max power transmission). Industrial chains are normally lubricated with oil sprays rather than grease. In my biking days, the recommended thing was Linklyfe, which was a very heavy graphited grease which you melted on the cooker, and then soaked the chain in. But real mans' (i.e. British) bikes didn't have chain cases except in a few wimpy cases, so you needed a lubricant which didn't wash off in the rain. I once did a particularly wet trip from London to the Peak District where I left home with the recommended half an inch of lateral movement on the chain, and arrived with 2 inches (the extra clearance being caused by wear in the pins and bushes).

[Dave Baker]

Chain drives should be relatively slack which I know is a meaningless term so I'll quantify it. Using general guidelines on motorcycle chains and automotive timing chains as a reference it equates to about 3/8" of slack (total up and down movement) of the lower run of the chain per foot of distance between sprockets. This should be done with the top run of the chain under mild tension i.e. rotate the rear wheel backwards lightly against the drive sprocket to take up any slack.

What you need to be asking is how did it become tight. Anything that wears or slips ought to make it looser.

Applying grease over a chain does very little to help it because to be effective the lubricant needs to get inside the links to do which it needs to be liquid. Motorcycle chain lubricant is either "spray on" which comes out of the can as foamy liquid and then soaks in and sets or "melt on" wax where you put a tub of the solid stuff on the stove, melt it, drop the chain in for a while and then take it out and let it drip dry and cool down. External grease just picks up grit and acts as an abrasive. Ideally the chain wants to be dry on the outside and greased on the inside.

Brings back happy memories of touring Europe in the 70s on my Honda 550 when I actually took my huge tub of Filtrate Linklyfe with me and did the chain half way round over the camping gaz stove.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "The Medway Handyman" saying something like:

3/8" play per foot will do as a starting point - you know when it's too tight for slack after that.

[Gib Bogle]

Ah, that takes me back. Put the pot of grease on the stove, go outside to work on bike while grease melts, get distracted ... AAARRHH! run inside, no grease in pot, black sticky soot over all kitchen surfaces, walls, ceiling ...

[PeterC]

Some slack is needed so that the bearings aren't overloaded and there isn't excessive friction between links and teeth. As has been said, a chain transmits force by pulling (a chain at work was set up to transmit by pushing - I was the only person who noticed the bleeding obvious), so there should always be some slack in the return run.

There's also cyclic variation in chain tension; this is caused by the difference in effective distance due to the position of the teeth and is significant on small sprockets - I'd guess that the scooter has v. small sprockets. This effect becomes hidden in other factors with bigger sprockets, so a 52 to 24 drive would hardly suffer it.

Now, there are a couple of other factors: concentricity of sprockets/bearings is often poor and the throw can cancel or reinforce as the sprockets turn and 'hunt' through their radii; a cheap chain can have slightly unequal links, especially if wear is partly due to poor materials, and this also can interact with the other factors.

I rode fixed wheel for several years and spent a lot of time adjusting one chain, then slung it and got a decent one. On fixed, it was necessary to find the tight spot (sometimes needed several runs round the whole chain to get the worst case) then adjust it to give some movement in the centre of the run. A tight chain did cause extra sweat!

[Mike Clarke]

But isn't that caused by slackness due to wear rather than adjustment? If the links and pins are seriously worn then the distance between the pins will have increased slightly with the result that as each roller arrives at the sprocket it isn't correctly aligned with the gap between the teeth and wears away the leading edge of the tooth.

[Lobster]

God, that takes me back. My mum used to *love* it when I did that...

David

[Lobster]

ROFL - memo to self: always read the rest of the thread before posting your own contribition!

[Bob Eager]

I was forced to do it on a Primus stove in the back garden...

[Andrew Gabriel]

Certainly, my experience on my bicycle is that the chain and sprokets wear together, and keep each other working together, long past the point where neither will work with a new companion. Result is that you either have to replace the chain long before you notice it slipping on the sprockets (measuring the stretch), or you have to replace both at the same time. Since my gear is all InterGlide which seems to be well obsolete now, I'm letting it all wear out before replacing the lot. (Got the last IG chain free because I was first person in years to ask for one, and the shop had had last one sitting in stock for years.)

[<me9]

There should be a little slack, but not enough for the chain to slip a tooth.

What!!!! What's wrong with WD40?