Centec 2a vertical head shaft bearings.

Hi. I've recently bought a Centec 2a which needs a bit of work. I've run into a bit of a problem with the bearings on the horizontal drive shaft within the main body of the vertical head. A v-belt that was too short (and so too tight) had been used, which I think has contributed to the rear bearing rollers being pitted and scored. I've got the shaft out no problem, but the rear bearing outer race is up a bit of a blind alley (I hope the 2nd photo will make it clear) with no way of getting a bearing puller onto it, and no way of getting a drift in behind. Any good tricks to get it out?

(sorry, but photos are on their sides!)

Also, does anyone know how to get to the front bearing? I can see where it is, but can't work out how to get there! There is a grub-screw on the top (see photo 1), and maybe one on the front face (although I can't find an allen key to fit it), but I can't see what they might release. Has anyone been here before and can offer some advice. I can't find a section drawing of the vertical head to refer to.

Many thanks.

Phil in Cornwall.

There was a section drawing on the old Centec site, but a bit of a sketch and not much use to you.

BTW don't be tempted to use that Dewhurst as an on /off switch it's not what they are good at.

You do need a reverse, but only when you switch from vertical to horizontal, otherwise you have to feed the table in the opposite direction to vertical milling.

Edited By Dave Halford on 05/05/2021 17:55:24

Hi

Thanks for your help Dave. I have managed to make a bit of progress today. Very carefully putting a tapered brass drift down the hole under the grease nipple moved the outer shell a bit, and then a couple of hours of making up some brass levers and using them etc got it out. For future reference, the bearing is a Timken, numbered 03062 on the inner race, 03162 on the outer. I think the front one is the same but I haven't cleaned the filthy grease off yet.

Better lighting revealed a second grub screw at a 9 o'clock position at the front, which allowed access to the front bearing. Just waiting for the parts now.

Is there a reason why Dewhurst switches aren't good at on/off?

Is there a reason why Dewhurst switches aren't good at on/off?

The contacts are not really adequate for inductive load switching.

You've done it now but it's not good practice to use a brass drift to knock out bearings. Any small bits of brass left from the drift and not removed completely will ruin your new bearings. Use steel as it's much less likely to chip and won't hurt a hardened bearing. This was a golden rule when I was an apprentice at British Timken.

Edited By RMA on 06/05/2021 08:10:12

It seems a bit daft given the switch has an off position marked, BUT as designed the contacts operate slowly rather than flick and that coupled with the inductive start load makes them prone to arcing damage.

If you don't have the horizontal arm you don't need the switch.

I have Dewhurst switches on several machines. Provided they are switched rapidly. arcing problems are minor. I have only ever burn't one contact out after nigh on 50 years of regular use. It took me 30 minutes to replace the contact as I had spare tungsten rod.

The presumed wisdom on this site re Dewhurst switches, is overdone, to put it mildly. Will the naysayers please send me their "not fit" for purpose Dewhurst switches.. I guarantee to find them a good home.

Andrew.

Correct!

Given a dismantled Dewhurst looks solid old-fashioned quality, it may be worth explaining in more detail what the problem is.

To carry a steady current switch contacts only have to have a suitably low resistivity so they don't overheat. The design of a non-breaking switch just has to bring two metal surfaces together over an area big enough to carry the current without getting hot, which is easy to do. This is what the Dewhurst is designed for,

What happens when switch contacts open whilst carrying power is much more difficult.

• If the contacts don't part simultaneously over their full area, all the current passes briefly through a reduced contact area causing local overheating, which may severe enough to damage the metal. The effect adds up and once damaged, the contacts wear out quickly.
• When two contacts carrying power separate slowly an arc forms between them. As an electric arc burns at about 8000°C and Tungsten boils at 5600°C, the contacts have a hard time, welding or vaporising! The best way of reducing arcing is build the switch to pull the contacts apart as quickly as possible. This is usually done with a pre-loading spring toggle mechanism that breaks contact with a snap action.
• As NDIY says, breaking an inductive load like an electric motor, is particularly demanding. In this case, opening the switch causes the magnetic field in the motor windings to collapse and generates a voltage spike of up to several thousand volts. As 5000V will jump an air-gap of about 0.3mm the switch contacts have to separate even faster. In jumping the gap, the spark creates a conductive ionised path which can carry and sustain an arc between distant contacts long after that caused by breaking a non-inductive current has stopped. Switches designed for breaking inductive loads typically add some way of destroying the arc by cooling.

Whilst classic British machine tools are mechanically excellent, the same can't be said of ancient electrics. Beware perished insulation, absent safety features like NVR, dubious earthing, leaky capacitors, and stuck centrifugal switches. The Dewhurst switch is an old fashioned design, expensive to manufacture, and a shade below modern safety standards. They're satisfactory for reversing provided the power is disconnected first, but their chunky look and feel misleads people into thinking they can be used as an ON/OFF switch. Unfortunately the switch inside is a basic rotary type, the contacts open slowly and they don't break cleanly with the contact faces parallel. Better designs were available at the time but more expensive. Modern* motor switches are all improvements and affordable too. I'd replace a Dewhurst unless it's essential the machine looks authentically old.

No doubt someone will be along to say their Dewhurst has been used continually since 1947 and is still perfect. Maybe, but dodgy Dewhursts are one of the most common problems reported by Myford owners...

Dave

* Modern in this case means later than about 1970!

Machine up a steel disc to fit into the bore of the outer shell of the bearing,tack weld it in place using arc welder,then use a stout steel rod down the spindle bore,beat it out. this allows a blow inline with bearing bore and the welding tends to shrink the bearing slightly and loosen it. Done this many times removing sleeve double row ball bearings from lorry steering gears,good earner until my friend who reconditioned motor parts bought his own welder.

Dave, your initial post re swapping motor direction would have been more pertinent for changing to vertical - when a reversed spindle rotation is a 100% no-no for vertical operation, whichever way the feed might be.🙂

I'm 99% vertical till I cut gears, hence the bias.

You've probably sorted it now, but two of the grubscrews (BA) are plugs for the grease way and there is one at the side which holds the flange for the spindle.

hex key sizes here:

Hex Key Sizes

Edited By Richard Millington on 01/06/2021 20:07:41

Back in the 70's the small firm I worked for had a Myford ML7 and a new Super 7, they both had Dewhurst switches and no other way of switching the motors on and off. The common no volt safety switches had not appeared that early.