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Calculate HPU Pressure & Flow requirements Cheat Sheet by

calculate HPU pressure and flow requirements
flow     pressure     requirements     power     unit     calculate     hpu     hydrolic


Most quick die and quick mold change systems use advanced hydraulic pumps to generate the required hydraulic pressure. Many press machine operators do not understand how important it is that a suitable hydraulic pump is used to control the system. With lower quality hydraulic pumps, when supplied pressure is insuff­icient dies and molds can get damaged as they are not securely clamped in place during pressing or injection molding operat­ions.

Before you select a pump for your Hydralic Pressure Unit (HPU), you must calculate your pressure and flow requir­ements. To calculate flow required by your hydraulic pump, you must know the size and velocity of the actuators it will power. It is important to calculate the maximum flow required during simult­aneous actuator operation, such as a motor and cylinder working in tandem. You must always factor efficiency in your calcul­ations, so as not to lead to disapp­oin­tment during commis­sio­ning.

Simplified motor flow calcul­ations

Hydraulic motor flow requir­ement is generally easier to arrive at than cylinders, which cycle with differ­ential volumes. Simplified motor flow calcul­ations are as follows:

gpm required = cid x eff x rpm / 231
cid = cubic displa­cement of the motor
eff = the motor’s efficiency rating
rpm = revolu­tions per minute
231 = constant repres­enting 231 cubic in. in one gal
Example: 2.5 in2 x 0.8 x 1200 / 231 = 10.4 gpm

Cylinder flow requir­ement

Cylinder flow requir­ement is more difficult because of multi-step math. We must first calculate two volumes: the volume of the cap side and the volume of the rod side of our cylinder:

Cap side volume = (πr2) x stroke
Rod side volume = cap side volume – rod volume (the space the rod takes up)
Example: 4 in. bore x 12 in. stroke and 1 in. rod
Cap side volume = π22 x stroke = 150.8 in.3
Rod side volume = 150.8 – (π0.52) x 12 = 94.2 in.3

Hydrolic Power Unit

Now Calculate the Volume

Now we must calculate the volume required to get the cylinder to stroke in the time we desire, let’s say 3 seconds in this example. First, we must convert seconds into minutes, which reflects our pump’s gpm descri­ption = 3/60 = 0.05 minutes to stroke.

gpm required = (V / T) / 231
V = volume
T = time
Example cap side = (150.8­/0.05) / 231 = 13 gpm
Example rod side = (94.2/­0.05) / 231 = 8 gpm


You’ll notice it takes less flow to retract the cylinder in 3 seconds rather than to extend this. However, if you must absolutely meet a maximum stroke time, you will need at least 13 gpm (more if you wish it to accelerate quickly). Bonus points if you can figure out how quickly the cylinder will retract with 13 gpm.

As I mentioned earlier, you must now add all the combined volumes together of every simult­ane­ously operating actuator to arrive at your ideal pump flow. You may find this is where you have to compro­mise.

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