Strostkovy
Titanium
- Joined
- Oct 29, 2017
Yes, safety is a major concern and the appropriate safety equipment will be calculated and installed, and verified by a third party.40 tons? At 1000 IPM?
I wouldn't have my body anywhere near that thing!
There is a reason press machines of all kinds have ridiculous safety, there are quite a few accidents involving them.
But anyhow, you already know that.
We had an older 150 ton press brake some years back. All hydraulics in that one, two rams and servo valves.
It kept alignment and precision between the two rams with glass scales, one on each cylinder. Actually worked pretty good, especially considering it was from the 80's.
But I keep wondering, do you really know what spec you actually want?
14" travel at 1000 IPM, that's full travel end to end in about 0,8 seconds!
With the weight of a structure that can support 40 tons over 4 feet and keep straight.
Well, I can tell you that you will need a lot more than 1 kW servos for this!
But, let's have some fun with some hydraulic calculations, imagining that we go all hydraulic. Whatever you choose the numbers will be roughly the same. I'm using metric since that's my region..
Using the smallest cylinders that can press with more than 40 tons at a not-to-high pressure, that's two 120/70 cylinders at 200 bar (2900 psi). That's giving us about 45 ton combined.
We want to keep the cylinders small so we can move with speed too.
Say that the structure weighs about 1 ton, we will need to lift that in the returning stroke. This limits the lowest pressure we can use.
We want to keep the pressure low for positioning moves to keep the power requirements as low as possible.That is in the region of 8 bar/116 psi for two 120/70 to lift slightly more than a ton.
We're looking for a velocity of 25.400 mm/min or 423 mm/sec for 1000 IPM. That velocity with those cylinders give us a total flow of 575 liters/min @ 8 bar.
Nice! We're also approaching max speed for most standard piston seals, which is around 500 mm/sec.
Now, a pump that gives 575 lit/min at 8 bar will consume about 7,7 kW, at 100% efficiency. A more realistic number is probably about 85-90%, so 9 kW.
As I don't know the velocity for the pressing move I can't calculate that, but I imagine that it is somewhat lower, but let's say 7 seconds for 4"
We'll definitely need to use a slave pump to get higher pressure at lower speed, so that sounds reasonable. As the pump is VFD driven we can over speed the motor during light load for better performance.
Those numbers give us 20 lit/min at 200 bar. That's almost 7 kW at 100% efficiency.
While we're at it, let's calculate some basic screw data too.
I found that Hiwin has a 48x10 screw that actually can handle (theoretically at least) the load. Load is 196.200 N/screw. That screw can handle 242.800 statically, but the next one down was way under the target.
Now, with 90% efficiency in the ball screw we will need about 340 Nm of holding torque. When I'm looking in BEVI's catalog for a 4 kW 3 ph motor with brake, I get the number 10-50 Nm, adjustable. And that's a 4 kW motor, not a 1 kW.
A 22 kW motor has 60-300 Nm, still 40 under what we need. I'm having a hard time believing that a servo motor has more brake torque than any "normal" brake, especially considering that servos usually have quite small diameters.
So there is that..
Let's look at positioning, as that will give us some better numbers, right?
Well, if we go with the one ton load of the frame again, we're looking at 4905 N per screw. That's 8 almost 9 Nm. That with a speed of 423 mm/sec gives 2538 rpm on the screw, or 2,3 kW.
Combined that give us 4,6 kW, so a bit better than the hydraulics, although some of this is due to rounding errors on my side.
One thing to note though, all these calculations are static, IE no consideration for acceleration or deceleration. Add that and power requirements will jump.
We have a hydraulic brake now. Downside is it is gravity fed for the rapids which has some downsides.
I really should verify the speeds. I think I am trusting too much on the claimed speeds on our current brake, which is what I'm basing my numbers on. It's very likely that 400 ipm is absolutely plenty if the annoying valve shuttle delays and terrible acceleration is gone.
If I were to go all hydraulic I would use a 5hp motor with a single gear pump. Either the large single acting cylinder would get the full flow and pressure, or it would be open to the tank and the flow would go to smaller rapid travel cylinders. That's a cheap way to get very high travel speeds, but still has the servo requirements. It is potentially possible to shut a valve at the exact right time based on an encoder to control the stroke, but I'm not confident that will be accurate enough. Worth investigating though. I have equipment I can test with.
As for screws, I have a lot of math to do there on viability. I'd actually love to make an all electric brake with big motors, but that's a huge cost and amount of engineering currently. I'm hoping to be able to use a ballscrew that isn't very back drivable and will not require much torque to hold at load, but I do not know if that is at all viable until I do some more calculations for the updated speeds and ram weight. So far it doesn't seem like it. An external brake is an options, but the entire concept relied on the nonbackdriveable screw to decouple the rapid and pressing mechanisms, and at a certain point it's not worth bothering with excess complexity to go with a simple mechanism.
