You may not be giving the designers of these machines enough credit for how strong they built them…
You are absolutely correct. I prefer to err on the safe side. It's in my DNA to do so.
I confess I'm a little leery of beating this subject to death. But I am genuinely concerned about adding faster braking to a lathe or adding a bigger motor WITHOUT suitable other mechanisms to manage those very significant increased loads. Doubling them as is often done is simply not a trivial change in my opinion.
I do have good engineering experience and knowledge to justify my caution. I know exactly what will happen to a vehicle transmission if you double the horsepower and I know exactly what will happen if you throw the transmission into reverse to brake the vehicle at full engine output - both assuming you can transfer that power to the road like you can to the work on a lathe.
Both are extreme examples. I only use them to make the point that gear trains are expensive and very seldom overdesigned all that much.
I'll also wager that the gears on your Colchester are much more likely to have a stronger safety factor than those on than my Taiwanese Lathe.
Although those of us who use our machines with care and within their limits never have a problem, we all know that lathe gears, shafts, and even boxes can and do break for any of a multitude of reasons - usually abuse. They are not infinitely strong. My guess is that gear sizes and strengths are simply chosen to exceed the requirements by whatever increment is available off the shelf on design. Therefore some parts will be much stronger than needed and some will only be marginally stronger. It's those marginal choices that scare me.
If you cut the design braking time in half, you double the load in the gear train. If you accelerate twice as fast, you double the design gear train load. That is a rather over-simplified but obvious reality.
That's why I'm cautious about doubling the motor size or reducing the braking time.
I think we can all agree that bigger chucks and bigger parts significantly increase the loads and therefore it isn't as simple as spinup or spindown time. But I am inclined to believe that we can think about those like multipliers that were also factored into the design.
With all that mombo jumbo as background, here is the rub and my real concern. Let's use your lathe as an example.
It is rather impressive that your Colchester will accelerate from zero to 1000 rpm in one second. Mine certainly won't.
Now let's swap that 8" chuck for a 12", and add a big flywheel that you are truing. Your lathe is definitely designed to handle that additional load. But I doubt your accel time is still 1 second. I bet it's more like 4 seconds or even more as the work load gets greater. That's because all the available horsepower is used while spinning up the lathe. There isn't any more hp available to speed up the accel time with the higher load. Same goes for braking time - it will increase with higher loads too.
However, the subject that I have focussed on with my cautions is the impact of adding a bigger motor or adding more braking capacity (via the motor) over the original design.
There is only one way (that I can think of) to do it safely. That is to arbitrarily increase the acceleration/decell time proportionally to the load in order to prevent the bigger motor/brakes from overloading the gear train.
Increasing/decreasing the time is easy. Proportional to the load is not. You can use a VFD to control the accell time of a bigger motor but to what? The same? I don't think so. Double? I don't think so either. I'd guess the same is fine with the no load, but prolly 5x is more realistically safer because of the possibility of higher loads (bigger chucks, heavier work). Essentially, the lathe was designed for a smaller motor and the safest thing to do is to simulate the smaller motor's accell time vs various loads. The problem is that a VFD (or other control device) does not know the load curve, so I think you have to use the worst case and set it up for accell/decell for the heaviest load you anticipate ever using. Same goes for braking.
Of course, that also means it's overkill for 95% of the real world uses. It's also gunna feel funny with a slower no load accell time with a bigger motor than was there before with the smaller motor....... but it is what it is.
If you decide to be aggressive and set all the times the same, everything will work just fine forever - PROVIDED there are no additional loads. But I'd highly recommend increasing these times to accommodate those times when you do have bigger loads.
Thanks for the opportunity to unload my brain on this topic. I've been thinking about it for a while now and needed to be able to lay it all out in this way for my own benefit as well as for anyone else who didn't bail out after the first few paragraphs.
Lastly, as an engineer, I tend to write things in absolutes even though they are really mostly educated opinions. That style of writing is also an outcome of my time teaching - so it's a double whammy. Please forgive me for that. In reality I enjoy and welcome debate and disagreement.
) on a 1953 Buick when I was burning rubber in front of the high school. And yes, the point of sudden reversal at full throttle will do it. Luckily the VFD is smart enough to slap on the brakes and stop, then reverse.
