Variable belt drive vs a gearbox

drogus · Mar 7, 2022

Hi,

I've been thinking about a design consideration for milling machines and lathes, specifically ways to change speeds. In CNC machines these days there is often only an electrical control of the spindle speed, but in many manual machines a gearbox or another mechanical transmission is used. I know that in many high end machines (Schaublin, Weiler, Aciera, Fehlmann, Hardinge and I think some other American toolroom lathes) a stepless belt drive was used.

Now, I'm no machine designer, so I'm clearly missing something, but when I think about it, the stepless drive has a lot of advantages with little downsides: it seems mechanically much simpler, it allows for a better speed control, it's usually quieter (or at least that's my experience). The only downsides that I can see are: lower range and efficiency, but the former can be countered by electrical gears on a motor and I don't think the latter is a big issue. For heavy machining I guess a belt maybe could slip easier? And then some manual lathes have a speed preselector, but that's rather rare.

Is there something I'm missing? Are there any other reasons why most of the Chinese milling machines for example have gearboxes? It seems to me that machining and hardening all those shafts and gears is harder than creating a set of pulleys?

Lewie · Mar 8, 2022

How many automobiles do you see with a "variable belt drive" ?? As I recall there has been exactly one in all history. :-)
...lewie...

howieranger · Mar 8, 2022

There are a number of cars out with CVT transmissions, my wife's Rav4 Hybrid has one. Granted the belt is actually a chain but the same principal I think.

Gordon Heaton · Mar 8, 2022

Lewie said:
How many automobiles do you see with a "variable belt drive" ?? As I recall there has been exactly one in all history. :-)
...lewie...

An automobile isn't a machine tool. A lot of motor vehicles use belts for variable speed, things like snowmobiles for example. Few machine tools need to transmit hundreds of horsepower. My mill uses a belt, and being able to achieve exactly the speed I want, and change it while running (even while cutting) is very convenient. It's also relatively trouble free, only requiring infrequent maintenance.

A VFD has all the same features of convenience, and may never need service so it wins in that department. Some of them are noisy, and machines so equipped lose power at very low speeds. I'll keep the belt on the mill, but would not hesitate to replace it with a VFD should that someday be necessary.

projectnut · Mar 8, 2022

What you're proposing has been done on a commercial basis over the years. I have 3 machines in the shop that accomplish speed changes through the use of belts and variable sheaves.

One example is the Sheldon MW series lathe. These machines were built from the late 1950's through at least the 1960's. They have a Worthington drive which is similar to a Reeves drive. The speed change in this case is accomplished by opening and closing the variable sheaves with a small gear motor. In essence all the operator does is push a button to either increase or decrease the speed and release when the tachometer reaches the desired speed.

A second example is the Bridgeport Series 1 milling machine with the variable speed head. The speed changes in this case the speed of the spindle is changed by turning a crank that manually opens and closes the variable sheaves.

A third example is a Jet JDP-125VS3 drill press. The speed changes on this machine are accomplished in the same way as those on the Bridgeport mill.

I think some of the main reasons these systems have fallen out of favor are the expense to manufacture and maintain. There are literally dozens of moving parts in the Sheldon system, and a gearmotor to power them. I believe it's far less expensive and considerably less maintenance involved in newer electronic systems.

Here are some pictures of the Sheldon system:

Vecair · Mar 8, 2022

Less vibration with belt drives. My Excello has variable belt drive, never had a problem and works perfectly. As noted can change speed while running and very quiet. As noted, motorcycles, cars, snowmobiles and plenty of equipment use variable belt drives. I vote for belt drive!

howieranger · Mar 8, 2022

For my mill the variable shiv drive works very well, (Reeves drive?) For the lathe adding a VFD for the 8 speed gear head had even more advantages. Being able to program the the RPM ramp up rate and dynamic braking and turning up the RPM while parting off is really nice.

PackardV8 · Mar 9, 2022

At the lower end of the machine tool spectrum, failure of the Reeves drive system has caused more Clausing/Rockwell drill presses to be junked than all other parts in them combined.

jack vines

beckerkumm · Mar 9, 2022

As stated earlier, a high quality Reeves or Variator system is expensive. There is a good write up on the Ruemema website about what they believe to be the advantages and disadvantages of the different lathe drive systems. Their preference for variable drive is the variator vs the vfd. I don't understand the discussion about torque at the motor vs torque at the tool tip but they are pretty high end rebuilders. I do know that in the old machine world, you want to be sure the Reeves drive is working when you buy as it is expensive or impossible to repair economically but considered a real benefit to the machine. I think Moore used a drive very similar to what Rivett used and both Moore and Rivett were considered pick of the litter. Monarch and Hendey went through a lot of effort to avoid the variable belt but Smart and Brown, Schaublin, and Leinen used a variable belt.

If anyone wants to expand on the torque at the tool thing, I'm interested but I may be a cult of one. Dave

svs · Mar 9, 2022

Horse power is a rpm times torque function. A VFD will hold torque constant so power varies with rpm.

A mechanical speed reduction (gear or belt) holds power constant so torque increases as rpm decreases.

FWIW-A modern John Deere combine puts 300 or more HP to the threshing rotor through a variable speed belt.

Sent from my iPhone using Tapatalk

CarbideBob · Mar 9, 2022

I think the OP does not own a racing snowmobile.
This was so great a use on B-port mill at the time and the best deal at this time.
You say "Now, I'm no machine designer"... well you may be on that road.

richard newman · Mar 9, 2022

My Hardinge DSM 59 lathe has a 2 speed motor and variable pulley drive, works great. The shiv is opened and closed by a separate motor, really easy and quick, and absolutely no vibration.

tylersteez · Mar 10, 2022

The industry I'm in has primarily used reeves and varidrive belt transmissions on the machines for the past 80 years. Modern technology has made them pretty much obsolete. They are reliable but expensive and damn near impossible to replace at this point in time. We have began retrofitting motor/gearbox/vfd's to replace them.

To be honest I absolutely dread it when the variable belt transmissions have issues. The VFD motor drives are problem and maintenance free aside from gearbox oil and the occasional VFD issue which are never too hard to figure out since VFD's will give you an error code. The repair never involves taking a big heavy greasy transmission off the side of your machine that is sandwiched up against a wall and rebuilding it on the bench.

I think of it similar to carburetors vs fuel injection. you wont be fiddling with your carb at 5am in the morning when its 20 degrees out just trying to get to work but you may have to replace a throttle positioning sensor or something relatively easy once a year at the most.

hanermo · Mar 10, 2022

VFD´s got vastly cheaper than the technicians hours plus mechanical pieces for any transmission.

My lathe has a 2.5 kW ac servo.
It delivers equivalent torque to a HAAS 12.5 kW (ST10) at 600 rpm (90 Nm peak, 12000 rpm, iirc).

It is mad to consider anything else, since ac servos can be mounted anywhere, anyhow, for not much money.
And they last for a long time, make no noise, little heat, and can be exchanged for a new one in minutes.

And ac servos are about 10x faster than vfds in everything, and maintain perfect cutting speeds independent of impact loads.
A VFD spins up a spindle (2kW) in about 2 secs.
A HAAS ST10 about 2-3 secs - you can adjust it.
An ac servo spins it up in 0.03 secs.

(If your mechanicals can take it. I use 30x less for a 1 sec acc).

The best car on the road is a Tesla 100 PD, delivering amazing acceleration.
From a VFD/servo combo they developed, over 14 iterations on the electronics, per 2021.
With no gearbox.
1400 amps peak at 440V, iirc.
On the older motor, pre the variable reluctance m3.

Anyway, the company with the most skill, experience, and money, and 1B$+ in RD per year does not use gears.
After 15 years practice and 1M+ heavy expensive cars on the road.
With stellar results.

drogus · Mar 10, 2022

Lewie said:
How many automobiles do you see with a "variable belt drive" ?? As I recall there has been exactly one in all history. :-)
...lewie...

That's a weird comparison, what does a car have to do with a machine? In my post I listed 5 companies that produced lathes and milling machines with variable speed drives. All of them more than one model, with some companies definitely having more than a few - like even Schaublin - Schaublin 13 and 22 milling machines and 125, 135 and 150 tool room lathes. The same goes for Fehlmann - it produced numerous manual drill presses and milling machines with a variable drive etc etc etc

drogus · Mar 10, 2022

projectnut said:
I think some of the main reasons these systems have fallen out of favor are the expense to manufacture and maintain.

That's the part I don't fully understand. I mean, yes, if the variable drive is controlled by a DC motor or hydraulically then I undersand it(now I realized one more company doing this kind of thing was Cazeneuve - the Cazeneuve 360 toolroom lathe had a hydraulic control for the variable drive and from what I heard it's infamous for leaking and breaking). But most of the machines I saw have some kind of a crank or a handle for control. Now I also remembered that one of the vertical bandsaws I owned had a variable drive too - a Startrite 316. It had a hand crank for changing the speed that was moving one of the pulleys.

With this kind of arrangement it doesn't seem to complex, but I will be doing more research on that, cause apparently it's not that simple.

PackardV8 said:
At the lower end of the machine tool spectrum, failure of the Reeves drive system has caused more Clausing/Rockwell drill presses to be junked than all other parts in them combined.

Interesting, do you know what was the reason of most failures?

drogus · Mar 10, 2022

hanermo said:
VFD´s got vastly cheaper than the technicians hours plus mechanical pieces for any transmission.

Sure, but my question only concerns manual machines with a mechanical transmission. I know that a VFD with a strong motor is much easier to maintain than a manual transmission, but I wanted to compare two types of manual transmission systems - a gearbox vs a variable pulley system.

To make it simpler let's consider two machines:

* an older version of a Flott M3 drill press - it has a variable speed drive with speeds up to 4000RPM. The speed is controlled by a lever on the side of the head. I attach a photo, I just sold this one a week or two ago
* a new drill press from China with a gearbox - similar capacity, speeds up to 3200RPM, but costs 2500EUR vs probably like 6-8k EUR for the Flott when it was new

Now, obviously, the overall quality of the Flott is much better than a Chinese machine, but I get that if a variable pulley system was cheaper than a gearbox, the Chinese machines would definitely be equipped with those. So my question is mainly: why is it more expensive to manufacture a system of two pulleys rather than a set of gears, shanks etc. Again, I know I'm probably missing something as pretty much everyone here says the same thing, but I'm wondering why. I'll do more research, but any insight from more experienced machinists would be awesome.

drogus · Mar 10, 2022

tylersteez said:
To be honest I absolutely dread it when the variable belt transmissions have issues.

I totally agree that a VFD is better than a mechanical system, but then a gearbox can break as well. And then repairing a gearbox is also expensive, especially if you can't order a replacement gear. But I guess what I'm asking is: what exactly makes variable drive systems more expensive, more error prone or harder to repair when compared to gearboxes (and *not* when compared to a VFD). Like: your boss says two machines came in for a repair - one with a gearbox and one with a variable pulley system. In this hypothetical scenario would you still prefer to fix the gearbox vs the pulley based transmission? And then why?

beckerkumm · Mar 10, 2022

It might also be worthwhile to distinguish between mechanical pulley adjustment and the power, hydraulic or electric, types used on high end machines. A reeves drive in a powerfeeder that turns with a knob to adjust is pretty cheap compared to the electric type on a Moore Jib bore. The mass of the Moore is huge. I would guess ( and only a guess ) that cost wasn't a consideration as the precision of the Moore required a belt drive to the spindle vs a gear to reduce transmission of gear vibration to the spindle. Some toolroom lathes isolate both the spindle and threading feed from gears to increase surface finish quality.

I'm wondering if there are design benefits to variator type drives vs gear on certain machines that manufacturers were trying to achieve rather than cost considerations. If that is the case, a comparison to a vfd assisted drive isn't relevant as the vfd is primarily a lower cost option with a different set of compromises. Dave

Variable belt drive vs a gearbox

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