While we at Sheffer don't provide hydraulic power units, these are invariably used in a majority of hydraulic applications to drive our cylinders. By looking at the system as a whole, in many cases hydraulics can provide a more cost effective solution especially when long-term energy costs are considered.
Many of our distributors can provide complete solutions and technical assistance regarding power units and a comprehensive hydraulic solution incorporating Sheffer cylinders.
The following is an excerpt of an email sent to a customer (edited in format only for readability) who wanted to know more about applying an existing power unit, and the relationships between speed, flow, and bore size of cylinders in their application….
Relationship between bore, speed, & flow:
Regarding the power units; it is quite possible that they could very well be running out of instantaneous flow capacity. As I’m sure they’re well aware of, punching and forming requires maintaining a certain amount of minimum speed as well as force to make things operate correctly. What would seem to be happening is that when multiple units are all energizing at nearly at the same time, the leading press reaches the workpiece, but has little available pressure because all available flow is being consumed in sending the other cylinders down. Only when the flow has stabilized will the pressure build back up. By then however, the first press may be simply resting on the workpiece or may have partially sheared it.
Of course larger cylinders require more flow. The flow is simply (effective area) * (cylinder speed). For example using the 6” bore at 1” per second:
Effective area = ((6 in)^2)*.7854 = 28.57in^2
(28.57in^2) * (1 in/sec) = 28.57 in^3/sec. Since 231 in^3 = 1 Gallon, then:
(28.57 in^3/sec) / (231in^3/gal) = .124 gal/sec.
(0.124 gal/sec) * (60 sec/min) = 7.4 gal/min. (GPM)
This is all instantaneous flow and only occurs during the actual cycle, so it DOES NOT necessarily mean that they need a 7.4GPM pump for each press cylinder!
1. Bigger power unit with more flow capacity.
2. Accumulators that can recharge between cycles but can help match and deliver higher instantaneous flow.
3. Add or adjust flow controls for each press being operated on the same shared power unit, to reduce maximum instantaneous demand flow for any given press. This sounds counterintuitive but may prevent the lead press from stalling out just when it needs pressure to perform the work. Note that this only works down to a point, since enough average speed must be maintained to give good results.
4. Sequencing the valves so that no more than ‘N’ number of cylinders are actuated at any given time.
Of all the solutions, they can all be made to work.
However, the FIRST thing I would look at would be the duty cycle. If there are large periods between cycles then #2 along with a variable displacement pressure compensated pump is probably the best solution from an energy conservation standpoint, since it will only deliver as much flow as necessary to maintain a specific pressure. It will then de-stroke to conserve energy if work is not actually being performed, or is being performed at a lesser rate.
On the other hand, if the work is all at a steady uniform rate, then a less complex (read less expensive) but physically larger pumping unit may make more sense. The accumulators may help with instantaneous flow, but be aware that they also increase the average pressure as seen by the pumping unit, since by definition they try to maintain a higher average pressure. That is probably why I tend to think variable displacement pumps when used with accumulators.
#4 is actually very effective when the working cycle is very short, since you then only have to size the required instantaneous flow for a predictable ‘N’ number of presses, and since the actual working time is short, the impact on production is still quite minimal.
Regarding reservoir sizing, there are rules of thumb relating to how long the fluid is allowed to remain in the reservoir, to allow trapped air bubbles to settle out before being carried back to the pump. I don’t have any of the actual calculations handy, and in fact this was usually left this up to the pumping unit vendors. The other thing that they look at is reservoir surface area, and the ability of this area to reject heat.
Obviously most if not all of the electrical power put into the system ends up as heat, either in the workpiece, but more likely in the hydraulic fluid. That in turn MUST BE rejected to the environment. Fan cooled radiators are also often used, especially if space is not available for a reservoir having enough surface area by itself. Fan cooled radiators can be more maintenance intensive especially in dirtier environments.
I realize that this may all sound more complex than they were wanting to think, however with the cost of power these days, solutions that use to be appropriate, ie., put the biggest power unit on it you could afford, could cost more in the long run due to all the energy consumption.
Hope this helps.
Are there any other solutions you would add, or additional food for thought on the solutions provided? Leave a comment below with your thoughts.
Image credit: jannoon028