Vital force

By Bruce Coons

Above: Figure 1. The Thompson friction welding machine spins one of the parts to be welded and then a hydraulic actuator brings the spinning component together with the stationary part. See full animation on

Electrohydraulic controller brings precision to friction welding process

March 2017 - When welding both aluminum and steel automotive drive shafts, aluminum parts in particular require careful coordination of process parameters to achieve high-quality welds while maintaining accurate part dimensions.

One prominent brand of friction welding equipment for these industries is Thompson Friction Welding, a part of Kuka Industries of Augsburg, Germany.

Friction welding is a bonding process that combines pieces of the same, or different metal, to produce a single unit. In a friction weld, no fillers are used and there is no melting of the material being bonded. Instead, the parts to be combined are brought together under a combination of force and relative motion. The ensuing frictional heat causes the material on one or both sides of the joint to become plasticized. As the process continues, the plastic material is extruded from the interface, carrying with it surface contaminants and oxides. Once the weld interface is clear of contaminants, the relative motion is stopped, the force is increased and the two pieces forge together. The resulting bond has strength equal to or greater than the base metals.

FFJ 0317 welding image1

Figure 2. The Delta RMC75E motion controller is shown at center left in the machine’s control panel.

Advantages of friction welding compared to conventional welding techniques include:

1. Defects associated with melting and solidification (e.g., porosity) are avoided.

2. Materials with different melting points can be joined.

3. There’s no requirement for fluxes or fillers.

4. High energy efficiency results as heat generation is confined to a narrow zone at the weld interface.

5. Production cycle times are shortened.

These advantages make friction welding an ideal process for producing high-integrity bonds in a variety of components, such as engine crankshafts and valves, trailer axles and rugged components for the oil and gas industry, like drill pipes and rods. 

Controlling contact

“We use position control to make initial contact of the parts. Then we back off a little and start the rotation of one part,” says Jeff Price, U.S. brand manager for Thompson Friction Welding. “When the rotational speed has been established, we use velocity control to move back into contact.” Subsequently, he says, “When sufficient material has been extruded, we stop rotation and switch to force control.” 

The extrusion process expels the impurities from the weld surface. Once complete, force control is used to forge the two parts together. To achieve consistent welded lengths, halting rotation and the transition to force control must be accurately synchronized. Delta Button Ad

In Thompson’s earlier drive shaft welding machines, the position of the hydraulic actuator and the pressure/force that it applied were controlled by a PLC that operated two separate valves. The dual-valve approach, and the controller they used, caused inconsistent time lags in coordinating the operation of the two valves. 

In other words, the PLC’s control loop scan time and the input/output scan time were asynchronous. This made it impossible to achieve the precise timing needed to achieve consistent length control (The PLC manufacturer did make available a dedicated hydraulic controller that would have avoided the timing problems but it did not have the ability to switch between position and force control).

The Thompson engineering team sought a solution that could apply more precise control of the process by using a single hydraulic valve. To do this, they needed a hydraulic motion controller that could switch smoothly from controlling position to controlling pressure/force. They chose the RMC75E electrohydraulic motion controller manufactured by Delta Computer Systems Inc., based in Battle Ground, Washington.  

“We have used the Delta product on several machines for different parts manufacturers,” Price says. “The main advantage of the Delta controller is its deterministic 1 ms control loop and its ability to switch seamlessly between position and force control. We need the controller to make very rapid control decisions at predictable times. One of the problems of using the other controller is a lack of consistent timing on the inputs and outputs.”  

Dual axis control

The RMC75E (see Figure 2 above) gets its motion commands over Ethernet from a PLC that performs overall machine control. Because the RMC75E is a two-axis controller, a single unit will perform all hydraulic control functions for Thompson’s new drive shaft welding machine, which is double-ended, in that it can weld two ends of a tube simultaneously.  

FFJ 0317 welding image2

Figure 3. Motion plot produced by Delta’s Plot Manager software. The horizontal axis is time. The blue plot line represents target position and the red line is actual position of the hydraulic actuator. The actual force applied by the cylinder is shown as the black plot line. The green plot line represents the control output to the single valve. The position-force transition occurs at about 6.2 seconds.

“In this application, precise synchronization of the motion of both axes is required because the force control phase needs to happen at precisely the same instant,” Price explains. “The force needs to be balanced between the two ends in order to avoid an out-of-balance resultant force, which could impact product quality and possibly damage the machine.”

To track the position of each hydraulic cylinder, the Thompson machine uses a linear magnetostrictive displacement transducer (LMDT), while the force applied by each actuator is calculated as equal to the difference between the hydraulic pressures measured by sensors mounted on each side of the piston.

The engineers at Thompson found programming the Delta RMC75E controller to be straightforward. Tuning the motion was simplified by using Delta’s RMCTools software, which is included with the motion controller.  

“I used Delta’s autotuning software for initial setup and followed by Delta’s Plot Manager software to manually make adjustments,” Price says. “It didn’t take more than an hour or so to get the machine tuned for proper operation.” 

Figure 3 shows a motion plot produced by Delta’s software. The transition between position and pressure control is clearly visible between 6.0 and 6.3 seconds into the cycle. The actual force applied (black plot line) shows a pulsing of force during the compression stroke as the metal deforms at the interface. The metal is made soft by the heat caused by friction, leaving it malleable but not molten.

The result is consistently good weld quality and better control of the finished length—a critical parameter with vehicle driveshafts. “The Delta controller makes it easier to ensure that the correct length is maintained within 2 or 3 mm,” concludes Price. FFJ

Bruce Coons is the regional applications manager, Delta Computer Systems Inc.



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