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Myford S7 Rattle

Hi all resurfaced again after a couple of days being poorly. Many thanks to all those who offered advice and help re my possible change to 3 phase VSD, I will be taking up the kind offers of help when I have sorted my current problem and decided what to do next. Before spending some money on a VSD for the Super 7, I thought I would ask some advice to make sure that it is worth the investment.

Some time ago it developed a rattle when "free running" at the two lower speeds of the high range (two pulleys nearest the rear of the headstock) at the time I thought the motor drive belt was shot and changed it temporarily for a link belt - noise disappeared. Having bought a new belt I replaced it with the standard belt and the noise returned. Yes I know I could go back to the link belt but I don't like "covering up" problems.

As far as I can tell it is coming from the backgear key/bronze gear attached to the spindle pulley assembly. I fiddled a 2 thou shim between key and gear slot and the noise stops. What I can't work out is why it is doing it with the standard belt and not the link type. To get movement in that area the spindle must be momentarily slowing and speeding up to hammer on alternate sides of the backgear key slot. I'm also intrigued by why it only happens on the two rear pulleys. If it is single phase pulsing why does it not happen on all pulley speeds? Could it be the rear spindle ball bearings which are obviously loaded more by the drive belt on those two pulleys? The spindle feels OK to rotate by hand but I can't think of another solution. Having thought about it as I write this I wonder if a change to smoother 3 phase would cure (mask?) this anyway? Still most machines do not make this noise so there must be something wrong.

Obviously, if I need to replace the backgear/pulley assembly (for which Myford will need at least one arm and leg) and the headstock bearings I will need to delay my change to VSD (orders from indoors). Any help, ideas or advice would be much appreciated I'm sure I can't be the only one to have this problem. The lathe is a late 70's power cross feed S7 although I don't think that makes a difference. The spindle main bearing is fine. Thanks in advance for your help.

Best regards

Easily confused of Wales

Reply to
jontom_1uk
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I get a similar noise from my much older S7 from time to time. It is dependenent on both spindle speed (I have a VFD fitted) and belt tension. I've put it down to manufacturing tolerances in the belt or pulleys, which cause the tension to vary very slightly in a periodic fashion, causing an oscillation which makes the backgear key chatter. Apart from the minor irritation caused by the noise, I don't think it's anything to lose sleep over.

Mike

Reply to
mike.crossfield

Mike

Thanks, it's always a comfort to know it's not just me. I hadn't thought of the belt manufacturing but as it now has two new belts you could have hit the nail on the head. Perhaps it will ease a bit when it has done more running. Another reason for why they might have changed to Poly-vee in the latest models.

I guess that if you already have a VFD you don't change belts that often anyway; do you leave it on any particular pulley? I suppose it depends on the typical size/material you are working with. Have you found the VFD to be a worthwhile addition? Everybody seems to recommend the change and tell me it's easier to accomplish than I fear. I shall just have to lever the wallet open again.

Best regards

Keith

Reply to
jontom_1uk

recommend

I find the VFD very worthwhile. If it failed I would certainly fit another. I use a 1/2 hp motor in place of the standard 3/4 hp single phase motor, and haven't noticed any loss of power. As everyone says, it's much smoother running 3 phase. Most of the time I leave the belts set for the highest speed on the low speed range, i.e about 700 rpm max. I've set the VFD to allow a low speed on this range of about 200 rpm - below this the power loss becomes rather noticeable. This seems to cover 90% of my needs. For screw cutting I usually swap to the lowest speed pulley on the low speed range, and hardly ever use the backgear. I've set the VFD for a slow speed ramp-up at switch on, and a fast stop at switch off. Both rather safer than the normal arrangement.

I bought my VFD from John Stevenson when he was selling off a batch of new/old model Jaguar Cubs a couple of years ago. It was very easy to wire up, and my only minor problem was interference on the workshop radio. I fixed this by putting a mains filter in the feed to the VFD (from memory it cost about =A33 from J A Crew). The motor was bought very cheaply 2nd hand, and rewired for delta connections - again, very simple. =20

hope this helps

Mike

Reply to
mike.crossfield

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