Press not reaching pressure - the BIG ideas

At some time in the life of your press you will wonder why your press is not achieving its rated pressure. We can almost be assured that this will happen to you, so do not be alarmed.

There are many issues that can cause the press pressure to decline over time or just stop building pressure due to a failure of a component. Here are some of the Big Ideas.

Small Load Prox Switch

Before we get too complicated, make sure that the Small Load Prox switch is still made. When not made we assume that the load is too small to fully pressurize. Small loads cause the diaphragm to hit the bed and creep under the can. So, we limit pressure if there is a small load. The small load prox is the second to the lowest on the prox tree. The setting for pressure if a small load is a config decision.

Don't believe a LED!

We spent a week and a half troubleshooting a press only to find out the voltage on the valve was 12VDC, not 24VDC. The LED on a coil connector will light at low voltage. So, while it is handy to see, the coil may not get the right voltage. Turned out that the power supply failed and was only outputting 12 VDC, not 24. Only after we changed a pump and a bunch of valves did we find the voltage issue! Do not repeat our mistake!
Measure the voltage on the coil at the coil pins to confirm the voltage!
Also make sure that the new coil you just bought is the right voltage. We have some 120 volt AC coils and some 24VDC coils that look identical except for the marking on the coil. The bypass valve coil is a prime example of this occurrence where you could order the wrong voltage since there are 2 choices. Check the markings before ordering.
A bypass valve and ram command valves are normally open. So, low voltage would not pull in the valve coil and create a low pressure condition.

Configure the Press Properly

We had a case where the customer configured a booster pump press as a non-booster press. The non-booster press does not use a bleeder valve and thus, never closes the bleeder valve. So, the press would pressurize to about 17 bar and bleed the Kawasaki to the tank through the bleeder valve (which is normally used to bleed the booster pressure back down to the Kawasaki pressures). If you have a booster pump press you must leave it configured as the correct model number. If you want to defeat the booster pump simply program pressures below the booster pump setting or take the power off the booster pump contactor coil.

The pump compensator spring will get weaker and require adjustment

When your press pump reaches pressure there are springs in the compensator of the pump that get compressed. Over time these springs collapse and require minor adjustments. This is not an IF statement...it is a WHEN statement. So, after your press has worked for a couple years it is almost guaranteed that the 2nd stage horsepower adjustment will have to be adjusted. Along with that you may have to adjust the first stage HP compensator, though this will be less likely to be noticeable. See the article on adjusting the HP compensators here (it's not quite ready but the manual shows the process...article being developed now).
The Linde pump is slightly different so read this article: Linde Hydraulic Pump 75cc

For an overview of how the press hydraulic circuit works see Single Stage Press Hydraulic Circuit - Ram

If replacing a Kawasaki style pump see this: Hydraulic Pump compensation - VDPP - Variable displacement piston pump

The thermodynamics of the press

When you create hydraulic pressure you can do work. And when the work is being done you are using the pressure. In the case of the press, we do work when we move the can up or down and move the ram up or down. Holding pressure on the ram does little work. Nothing is moving and no fluid should be moving. So, just a little HP is required. Normally at idle pressure, the pump motor runs at about half of full load amps.

BUT, when we move fluid at high pressure and we do no work (nothing is moving) we create heat in the oil that we poured into the motor electrically. Energy that goes in must come out. And if no work is done heat is created. So, why would you move fluid at high pressure when no work is being done? ....because YOU HAVE A LEAK.

Remember that if you are burning HP and doing no work, you create HEAT.
The cooler is rated at about 17 HP (for the 50 and 60 kg press). So, if you have a leak (burning HP) the fluid temp will climb.

Now you may ask, can I live with a small leak at the piston (for example)? The answer is yes. But, when the leak is greater than the HP of the motor, then the pressure will start to drop when pressing. Then you need to decide how bad it gets before rebuilding the ram with new seals. If your dryer line is overtaxed then this may be an easy decision.

The ram piston seals will wear out.

During the life of your press you will replace piston seals on the main ram. As the main piston seal wears and ultimately fails to seal, the pressure on the ram will be reduced. We have seen piston seals completely missing in some presses. These disintegrate over time and the pieces get transported into the tank filter. Fragments are left behind in the piston but they cannot seal the fluid in the ram.

Now a few interesting comments:

Wear bands act like a seal (but not a great one)

The wear bands on the piston actually create a partial seal. We have seen a piston seal missing and still able to pressurize at 35 bar on a 50 bar press. So, even if the piston seal is gone, the press still presses. It cannot press at full pressure because the leak is greater than the pump can feed with limited horsepower.

The biggest reason to replace a piston seal

So, what does a few bar mean, you ask. Not much. But, the difference in pressing at 50 or 35 bar could mean a lot of excess moisture that the dryers and ironers have to evaporate. So, lower pressures cost money measured at the gas meter. Pictures of ram piston seals and wear attributes

To test for a piston leak you can open the ram up piping ball valve and see if fluid flows out when pressing down. Alternately you can press a load manually for five minutes and while holding the down button have someone remove the coil wires off the ram up poppet valves. If the pressure pops up immediately on the ram gauge (showing pressure at the bottom of the ram) then you have a leak. Immediately put the coil wires back on.

Pumps:

All presses have Variable Displacement Piston Pumps. 2 types are used. The Kawasaki style is limited to 4600 psi while the Linde style is not limited. The Linde pump replaces both the Kawasaki and booster pumps on higher pressure presses. So, be sure to know which type you have. The Linde requires a relief and this relief is like a booster relief. Be careful! And see the note on the booster relief below.

The Kawasaki pump will run a motor to full load amps if there is a leak of pressure.  this is true at low pressure and high pressure.  
The Linde pump uses an input from the system pressure for the load sense (HP compensation) and if the system has a leak and the pressure at the SYSG point is lowered, then the pump will not demand horsepower and run at low amps.  Like, a bypass valve stuck open will only cause a draw at 20 amps on a 30HP motor while full load amps are 40.  So, this may confuse you into believing you have a proportional valve issue when you really have a leak to tank.

Valves fail to close and create leaks to the tank.

Any time fluid leaks to the tank the pump must feed the leak. The pump is HP limited to the max output of your pump motor. Because of this, we normally ask the question "what is the amp draw on the main pump motor when pressing?".
If the amps are high, this is the indicator that you are feeding a leak.

A press with good seals and no leaks to tank will run at about 2/3 of the full load amps of the motor when pressing. When building pressure the motor will work harder, but after pressure is achieved, the pressure compensator will limit horsepower by de-stroking the pump. We do not need any flow, thus we do not do any work, so the HP is limited

A few examples of leak possibilities:

System relief

A system relief is normally set above the max output of the pump. The system relief is a safety. It should never pass fluid. But, if set too low it will leak fluid to tank and the result will be excess HP consumption and heat. So, if your oil is too hot, it might be the system relief bleeding to tank while pressing. To test it, turn the system relief CCW and see if the pressure goes down further. If not, then turn it CW back to where you started and go look for a leak elsewhere.

Bypass valve not closing

The bypass valve is a safety that allows pressure to build when the ram is in the can. Otherwise the membrane could explode if you double loaded the can and pressed the goods and the membrane is not contained by the can.

If fluid is bypassing you can hear fluid flow through the hose to the tank from the bypass valve. You can also unplug the bypass valve when pressurizing to hear the fluid flow when testing the system. If not able to get to pressure you can unplug the valve to see if the pressure drops further. This will also help indicate if it is not closing.

We know that if the bypass is open when pressing you will get about 2 to 3 bar of pressure on the display. This has to do with the bypass orifice and the flow of the pump and the HP limiter. Trust that the motor will run at the 1st stage HP limiter setting of the pump if this valve fails to close. If you see 2 to 3 bar, go immediately to the bypass valve.

Prefill valve fails to close.

If the prefill valve fails to close, you will see "0" zero bar. The leak is so big that the pump cannot generate any pressure. The pump will run at the 1st stage HP limiter. This condition is usually an obstruction in the valve. See this article: Press not reaching pressure - Obstruction in the prefill valve.
We had one press that would go to 17 bar. On inspection, the prefill disc (the 4" seat) would spin by hand. It should be tight and take over 10 pounds force to push it open. So, while this is a rare find, it has happened. And on another note...do not trust that removing the gooseneck plug for oil leaking past the prefill valve is a good test. You would think it would bleed a lot of oil if the prefill was leaking but we proved otherwise and missed the leaking prefill valve for days.

Booster pump bleeder valve fails to close.

A bleeder valve or decompression valve is used on the booster pump press. This decompresses the press below the booster pressure. If the press is pressurizing at around 20 bar it could be a bleeder valve leaking, even when just using the normal Kawasaki pump and asking for less than the highest pressure. The pressure may rise if the booster pump is enabled. To test the idea, you can unplug the bleeder valve coil when pressurizing to see if the pressures emulate the failure. Check the voltage to the coil and the relay on the board controlling the bleeder valve before just replacing the valve.

Ram Command System

If you have Ram Command (soft squeeze) then you have 2 possible leak paths. We have seen pressure when these fail at 0 to 5 bar on the variable displacement pump and as high a s 16 bar if you have a booster pump and the booster is on. Ram Command is designed to relieve the ram pressure applied at the top of the ram while pressing against the relief at the bottom of the ram. You can actually "float" the ram with these adjustments to prevent the full weight of the ram from hitting the goods on a downstroke. For more on how this works see Ram Command - Soft Squeeze operation for sensitive goods

Booster relief

If you have a booster pump, there is a relief valve for this part of the hydraulic circuit. This valve is always in the ram down circuit even if you are not using the booster pump. Now, this is a dangerous topic. The booster pump relies on this valve for max booster pressure. The booster pump is a positive displacement pump. And the pressure can run as high as 10,000 psi. So, if you mess with this relief and turn it up too high, you can blow the rods out off the press. So, do not adjust this valve CW unless you are sure that this is an issue. And if you do turn it up while testing, turn it down again before you make another move.

Example, let's say you adjust it up and still have a leak elsewhere. You then find the other leak and when you pressurize you get a higher than designed booster pressure. BOOM. You can break a rod. Then you have a down press and a scared workforce.

Other Possibilities:

While it is not a leak per se', the directional valve may not be opening. The ram down side of the directional valve could not open intermittently. This could be caused by an output board issue where the contact is bad and intermittently not making contact.