Brakes

[Steve Maskery]

Hi all,

Can someone more knowledgeable than I am please explain how machinery is braked? Apart from my spindle moulder, which I can't fathom, all my machines are too old to be braked. How does a tablesaw stop inside 10 seconds? Or a bandsaw? Is it an electric brake on the motor or a mechanical brake on the blade (or wheel)?

All input gratefully received.

Cheers
Steve

 

[Chris Knight]

Steve,
Modern saws are electrically braked. I am not sure of the circuitry involved but there is a feedback into the motor windings that slows the rotation after you have hit the off switch. On my TS4010 there is a small potentiometer reachable through a small hole in the switch housing with a slim screwdriver that sets the time or current (not sure which) that goes into braking, so that the blade stops in a longer or shorter time.

 

[Scrit]

Can someone more knowledgeable than I am please explain how machinery is braked? Apart from my spindle moulder, which I can't fathom, all my machines are too old to be braked. How does a tablesaw stop inside 10 seconds? Or a bandsaw? Is it an electric brake on the motor or a mechanical brake on the blade (or wheel)?

Hi Steve

There are four main types of brake in use on "standard" woodworking machinery:

1. Hand- or foot-operated mechanical brake with brake linings. Very similar to the handbrake on a car and generally found only on narrow bandsaws and spindle moulders, although some Wadkin routers (the F-head ones) have a mechanism in their heads as well as some single-end tenoners

2. Automatic mechanical brake. Similar to (1.) above but the brake is pulled on under spring pressure and held-off when the electrical supply is on and the E-Stop or safety release is not active. Found on machines such as the Altendorf panel saw

The above methods require the brake, cable and linings to be inspected periodically and be adjusted (pain!)

3. DC injection brake. In these systems a device is fitted together with switch gear which means that when the E-Stop is hit the power to the motor os cut (it is a "self-holding" solenoid circuit) and DC current is fed into the windings which destroys the magnetic fields and causes the motor to "freeze" fairly quickly as the rotor and stator try to align (having now got dissimilar polarity). There are various combinations of this system and mechanical brakes as well as systems which monitor motor rotation and cut the DC injection once the motor has stopped turning.

4. Belt/pulley friction (a cheat). Some manufacturers (including MiniMax on some of their saws) depend on the fact that there is so much belt/pulley friction in their drive trains that the motor/cutterblock combination will halt within the required 10 seconds without the addition of a brake. With these you need to keep the belt tension high - just don't expect the belts or bearings to have a long happy life.....

I'm no electronics engineer, but those "crude approximations" are how I for one understand motor braking to work for the majority of woodworking machinery.

As to legality, well you know where to look..... A lot of the hand/foot mechanical brakes are no longer acceptable in law for training/commercial installations the exceptions being spindle moulders and (with an appropriate safety assessment) small band saws and single-head pin routers - and even that may be subject to HSE intrepretation inthe event of an accident.

Scrit

 

[Steve Maskery]

Chris, thank you; helpful and prompt, as ever.

Scrit, your knowledge never ceases to amaze me! How come you know so much about every aspect of everything???!!!! It's not a complaint! Thank you very much.

Cheers
Steve

 

[neilc]

Scrit, your knowledge never ceases to amaze me! How come you know so much about every aspect of everything???!!!! It's not a complaint! Thank you very much.

Steve you beat me to it. As I was reading Scrit's post I was thinking the exact same thing. It's amazing to me how Scrit has amassed so much knowledge about everything woodworking. It's great, long may you continue to post here Scrit =D> .
Neil

 

[Mike-W]

Steve

I purchased KDR 12” Planer Thicknesser last October, it has a cutter block of 105 mm x 310 mm. When the power is cut there is a lot of mass to stop in the required 10 seconds. Anyway when I installed it and run it up I found that when I hit the stop button I could hear a pulse of about two every second for about 14 seconds, I was a bit concerned about this as it also caused my workshop lighting to flicker slightly. I spoke to my supplier (Southern Woodwork Machinery) they checked with a Sparks and said it was the way the PT was designed to come to a stop. It needed this pulsation because of the mass of the cutter block.
I am no expert on electronics but I have come to the conclusion that there is an injection of current into the motor effectively reversing the direction of rotation to slow down the cutter block rotating on my KDR PT.
Mike

 

[Sonic]

Scrit (as always) is quite correct. If you connect DC to a rotating induction AC motor it will stop, pretty damn quick. This is why I imagine that the current is only injected in pulses, else some damage might be done.

The reason I know this is that many years ago I designed a rig for testing motor switching modules used in electronic petrol pumps. The way it worked was to have two of the AC motors "back-to back" connected with a drive belt. One would be switched on, and run for a minute or so, driving the second motor as a (not very heavy) load. It would then switch off, and just before it stopped rotating, the other other motor would switch on and drive the first motor. The idea was to have a slight inertial load at switch on to stress the drive circuits a bit. This worked fine, until one day one of the electronic switches (a device called a Triac) failed, but not completely. It normally either conducts in both directions (and so passes AC) or it blocks both ways. In this case the Triac started to act like a diode, and so injected dc into one of the motors, while the rig was going full-tilt. The motor was ripped out of the chipboard it was bolted to. So DC braking is very effective.

Obviously universal (brush-type) motors will run happily on AC or DC, so I imagine that rather more cunning is need to brake them - probably disconnecting the power source and using regenerative braking.

Colin C