The board and saw positioners on this edger board were moving too slowly and raising alarms because they were not set to the desired position within a specific time period.

The heat on the accumulator's shell was exceedingly high, showing that the bladder was being compressed too much from dry nitrogren precharge being too low.

Q: Our edger board and saw positioners are moving too slow. We have just changed the pump but still have the same problem. Can you help us with this as we are getting "positioner alarms" and they are slowing down production?
Bill R.

A: There are four board positoners on the edger infeed table that position the board prior to feeding it into the two edger saws. If these positioners do not set to the desired position within a specific time period, a "positioner alarm" is indicated on the operator's control screen. The edger infeed table chain must be slowed down to continue operating.

Upon arriving at the plant and surveying the system, I located two accumulators in the system. One accumulator supplied oil to the four board positioners, the other to the two saw positioners. The accumulators are used to supply oil at a high flow rate to the positioners when required.

The first test was to check the heat on the shells of the accumulators. If they are operating properly, the lower half or two-thirds of the shell should be hotter than the top half. The friction of the oil in the shell causes this heat as the accumulator discharges and is refilled. The heat on the saw positioner's accumulator was 105° F all over the shell. This indicated that the bladder-was being compressed too much as a result of the dry nitrogen pre-charge being too low.

The hotter portion of the board positioner's accumulator went about three-quarters of the way up, again indicating an undercharged condition. When the plant went down for a shift change we installed a charging rig with a gauge to determine the nitrogen pre-charge.

A block underneath each accumulator contained an automatic hydraulic dump valve. When the pump is turned off, these valves open and dump the pressurized hydraulic oil back to tank. When checking the dry nitrogen precharge, hydraulic pressure should be at 0 psi.

As a safety note, the hydraulic gauge should be checked to verify that the pressure has dropped to 0 psi prior to working on the machine. If this is not done, severe injury or death can occur.

With the charging rig and gauge installed, 400 psi was found in the saw positioner's accumulator. When the board positioner's accumulator was checked, it indicated 900 psi. To determine the proper precharge, the maximum system pressure should be known. This was determined in this system by the pump compensator setting.

The compensator in this system was set to 2950 psi; however, the recommended setting was 2600 psi. Accumulators that are used for volume should be pre-charged to half to two-thirds the maximum system pressure. The proper precharge for these accumulators with a 2600 psi compensator setting should be 1300 to 1716 psi. We attempted to precharge each accumulator to 1300 psi. Because of the low nitrogen in the existing bottles only 1200 psi could be reached. The pump was then turned on and the compensator adjusted to 2600 psi.

Upon checking with the operator after the plant started back up, he said there were no more position alarms on the edger. This also allowed the infeed table speed to be increased, raising production levels. I recommended that both accumulators be precharged to 1300 psi when the new nitrogen bottles were received.

What occurred in this system is typical of many circuits. First, the pump was initially changed without any checks being made. Second, the pressure setting on the compensator, 2950 psi, was turned up to very near the maximum rating of the pump, 3000 psi. This high pressure increased the system heat, shock, and current draw of the motor. Third, the nitrogen precharge had not been set up to be checked regularly. The precharge on bladder accumulators should be checked twice a year. Over a long period of time the nitrogen can seep through the rubber bladder. If a speed problem develops, the precharge should be checked immediately.

A preventive maintenance schedule should be set up for each hydraulic unit in the plant. This can improve the operating efficiency of the system and maximize production levels.

Troubleshooting Casebook is a regular column in our MRO department in which C.A. (Al) Smiley Jr. answers questions. Smiley is the founder of GPM Hydraulic Consulting Inc., a training and consulting company in Monroe, Ga. Smiley has nearly 30 years experience in the hydraulics industry as a distributor and educator He earned his bachelor's of education at University of Mississippi and is a certified fluid power specialist. Visit www.gpmhydraulic.com for more information.