Timing belt centered on the sprockets?

[Zeyneb]

Hi there,

I am replacing a timing belt on my car. It’s an inline 4 cylinder engine. It goes pretty smooth. The belt system is equipped with an idler pulley and a tensioner pulley. At this time I haven't tightened the bolt on the tensioner pulley yet. And now the belt runs on the very edge of the camshaft sprocket. I hope when I have tightened the tensioner pulley down and put a ratchet on the crankshaft the belt find its way to the center of the camshaft sprocket.

But in general, I am wondering what design feature keeps the timing belt centered on the sprockets? Have these sprockets a very slight V shape to keep them centered or so?

Regards, Zeyneb

[thm_w]

Usually one of the pulleys or idlers will have a flange, this will prevent it from running off the side. Its not V shaped.

[David Hess]

The ones I have worked with all had flanges on one or more of the pulleys.

[TERRA Operative]

90's Toyotas with timing belts have the crank pulley flanged, it keeps the rest in line.

[BrokenYugo]

The couple I've done had flanges on the crank pulley.

Don't forget to roll it over two turns after letting the tensioner go and make sure the marks line up again, it's easy to slip an extra tooth of belt in to the tensioned side of the belt and end up with the cam a tooth late.

[james_s]

Volvo uses a flange on the back of the crank pulley, I think that's fairly standard. The rest depends on the precise alignment of the other sprockets. You want to use your fingers to get the belt reasonably centered before releasing the tensioner, then rotate the engine in the correct direction by hand for 2 or 3 complete rotations to make sure everything is lined up properly. I use a ratchet or breaker bar on the crankshaft pulley bolt to do that.

[BrokenYugo]

Since the cam spins at half speed in a 4 stroke engine you have to turn it over an even number of revolutions to get the timing marks lined back up, 3 will put the cam mark 180 degrees out.

[james_s]

Well if the timing marks are lined up after the first two revolutions they'll stay lined up. I was more talking about the alignment of the belt centered on the sprockets.

[Carel]

The reinforcement of the belt (steel, fibre, whatever) is spirally (endless) wound. So the belt will always try to wander off to one side, depending on the direction of  rotation. A timing belt construction will always have features to keep a belt in its place.

[Zeyneb]

Thanks for all your contributions. Tightening down the tensioner bolt moved the tensioner slightly further towards the engine. Then rotating the engine on the crankshaft moved the belt in the right direction on the camshaft. But it is still pretty much riding on the outside edge. Maybe I will put some flat washer in some day, to have the camshaft sprocket stick out more.

It is a Toyota 4E-FE engine and the timing belt does additionally drive the oil pump. Not the water pump. The water pump is driven by an auxiliary belt. I noticed the oil pump sprocket had a flange on the front side. And the crank on the back side.

Anyway I installed it correctly regarding the timing and drove it back home from the DIY car repair shop, so that is nice.