Diagnosing electrical/hydraulic problems in a CNC tube bender

May 06, 2023

Melted connections at the fuse were a telltale sign than something in the system was being overloaded, though why it happened was unclear at the outset.

A truck and bus exhaust manufacturer was losing production time on a 6-in. tube bender because its main fuses were blowing out regularly. Pictures of the electrical cabinet revealed that the insulation on a couple of wire connections also had started to melt.

This was a serious situation.

All CNC tube benders have multiple systems that work together to drive the machine, ultimately forming straight tubing into the desired shape. This particular machine contained hydraulics for clamping and axis positioning; servo electrics for axis positioning; single-phase, 120-V AC control voltage; and 24-V DC control voltage. The main electrical system provided power to drive or control the other systems.

(As an aside, there should be a disconnect between the machine and the building's source power. And between the disconnect and the rest of the machine, or as part of the disconnect itself, there should be some way to automatically remove power if something on the machine fails and causes electrical current to spike.)

A typical CNC bender uses three-phase power, meaning it has three current-carrying circuits supplying power to the machine, so there will either be three fuses or a three-pole circuit breaker in the disconnect circuit. Power gets distributed to various parts of the machine from there.

Inside this particular machine's electrical cabinet, the wires with melted insulation led to two small step-down transformers that changed the main power supply from 480-V, three-phase to 120-V, single-phase control voltage.

One transformer supplied power to the various directional valves on the hydraulic system. The second transformer was smaller, supplying the 120-V, single-phase voltage that, in turn, powered a DC power supply that provided 24 V to power the control PC and I/O system.

Each of these transformers was protected by its own set of fuses, but these fuses were not being activated when the main fuse blew. In electrical circuits, circuits in parallel will have common voltage but uncommon amperage. In other words, while each of these transformers was supplied by the same 480-V circuit, each consumed different amounts of power in its operation. Using a clamp meter—a common type of test meter that can measure the current of an AC circuit, without touching a wire, by measuring the magnetic field as electricity moves along the wire—it was quickly determined that neither transformer was drawing enough current to cause a problem with its own small fuses, and certainly not with the main fuse.

The same electrical power bus that supplied the two small transformers also powered the motor driving the pump for the machine's hydraulic system. Using the same clamp meter amperage, the draw on each leg of the pump motors was tested. With the machine started and nothing moving, each leg was drawing only about 35 amps—not enough to cause a 100-amp main fuse to blow. However, as soon as an operator selected and moved a device causing the hydraulic system to jump from low to high pressure, the meter displayed 103 amps.

The electric motor driving the hydraulic pump was controlled using a motor starter. A motor starter is a combination of contactors and a thermal overload relay. When properly wired, the overload relay will interrupt the control signal to the contactors, causing the motor to stop when it gets overloaded. However, because this relies on heat generated by the motor overloading, a motor starter will allow a motor to run for a very short period of time in an overload condition until the thermal overloads get hot enough to open the control signal.

In this particular operation, a compensator was leaking at the pump.

After confirming that the motor starter was set correctly according to the motor's maximum rated amperage and that the overload relay was correctly wired to interrupt the signal from the control system, the technician, Al Drinnon of RbSA Industrial, had to switch gears. What started as troubleshooting an electrical problem now looked like a problem with the hydraulics.

Tube benders regularly use hydraulic systems because hydraulics can economically deliver the large forces required to deform a tube into a shape. However, hydraulic systems create different challenges for maintenance, troubleshooting, and repair.

The hydraulic system on the malfunctioning machine was powered by a pressure-compensated pump, which is very common for tube bender applications. The pressure compensator automatically reduces or stops the flow of hydraulic fluid if pressure rises above a preset maximum (often called the firing pressure). The compensator prevents the pump from getting overloaded. Most hydraulic industrial equipment is designed to work between 2,000 and 3,000 PSI. When most pumps fail, pressure drops as fluid leaks out. But when pressure-compensated pumps fail, they build up too much pressure and fail high. A relief valve on the hydraulic system will prevent the pressure from building up too much by returning liquid to the tank when there's too much in the system.

The hydraulics systems on most tubing benders are designed to operate at 2,000 PSI or less, and the relief valve is usually set 300 to 500 PSI above that. Drinnon noted when the machine was turned on with the pumps running but idle, there was less than 100 PSI displayed on the pressure gauges, but when a device was moved, pressure would rise to 2,500 PSI.

There is a valve that routes the pressure line flow back to the hydraulic tank to prevent pressure buildup at idle. When a device is moved, that valve blocks flow to the tank, allowing pressure to build for machine operation. This is called system pressure. Drinnon was seeing normal operation at low pressure. In system pressure, the compensator was building too much pressure, which was overloading the electric motor turning the hydraulic pump.

As it turns out, the compensator was leaking at the pump. Over time, system pressure would decrease, reducing the machine's ability to create hydraulic force. So, maintenance personnel would adjust the compensator to bring system pressure back up to the normal operating range. Eventually, the leak at the compensator was bad enough that it was removed and inspected. Finding that O-rings had failed, maintenance personnel replaced them and reinstalled the compensator.

"After repair of a pump or compensator, prior to restarting the machine, the system should be adjusted so pressure will be at or near its lowest setting," said Gary Moore, operations manager at Air & Hydraulic Equipment. "This is done by turning the adjustment screw on the compensator counterclockwise until it is adjusted nearly fully out. Do not remove it. The pump can then be started and set to build system pressure. The compensator's adjustment screw should then be turned clockwise until the system is at the desired operating pressure."

After the leaks at the compensator were repaired, the system had built so much pressure that it was being reduced through the relief valve at 2,500 PSI. Drinnon also adjusted the relief pressure down to the machine's specification of 2,300 PSI.

"To adjust the relief pressure, I turned the relief valve's adjustment screw fully in to its highest pressure," Drinnon said. "Then, using the compensator, I set system pressure slightly higher than the designed relief pressure; because the relief was to be set at 2,300 PSI, I set the compensator at 2,400 PSI.

"Then, using the adjustment screw on the relief valve, I reduced pressure until it was at 2,300 PSI. Last, I set the correct system pressure of 2,000 PSI using the compensator. Now if the compensator fails, the relief valve will prevent pressure from exceeding 2,300 PSI."

Finally, the wires with the melted insulation were clipped back and new connectors were installed. Now the machine is back in service.