Maintaining Servo-Driven Presses

May 22, 2019

Servo-driven presses offer improved control during the feed cycle while allowing for the stroke to be adjusted to allow for long feed lengths and the manufacture of complex stampings. While the investment on the front end runs higher than with standard mechanical presses, the rewards are worth it—so long as you understand the technology, operation and maintenance requirements. Armed with this knowledge, metal formers know what to look for and what to ask as part of an ongoing maintenance program.

Let’s learn.

Structural Features and Frame Design

Some manufacturers ensure press longevity by performing finite-element analysis (FEA) on the press frame, drive components and slide assemblies to optimize construction and prepare the frame for dynamic forming processes.
Similar to a standard mechanical press, the frame of a servo press and slide assembly provide mechanical rigidity and resistance against deflection during the cycle. Most servo presses use a steel-plate-welded assembly with power generated by servo motors in various configurations based on the required press force and forming task. Low-stress annealing of the welded parts has become standard. Some manufacturers ensure press longevity by performing finite-element analysis (FEA) on the press frame, drive components and slide assemblies to optimize construction and prepare the frame for dynamic forming processes.

Drivetrain Deciphered

The use of dynamic torque motors in servo presses delivers several advantages:

  • High torque available at low speeds;
  • Low moment of inertia, very dynamic;
  • Extremely quiet as compared with standard mechanical presses;
  • Temperature-controlled, water-cooled;
  • Shorter brake angle;
  • No backlash or rotor losses;
  • Nearly maintenance-free operation; and
  • Long production lifetime.

Most servo-driven presses still use a gear reduction, particularly on larger two- and four-point presses. The drivetrain, characterized by one or more torque motors attached to the drive shaft and the absence of a traditional flywheel assembly clutch/brake combination, ensures high dynamics, efficient processes and reduced maintenance due to the absence of these mechanical-press features.

The servo drive, which controls press movement, allows the press drivetrain to move in either direction during the stroke. Photo courtesy of Simpac.
As for the braking action, that’s performed by the motor itself. Presses with higher tonnages and correspondingly heavier slides have an additional hydraulically released and spring-loaded holding brake specially developed for servo presses. This prevents slide movement during drive inactivity or a press-system failure.

While drivetrain designs from different manufacturers vary slightly, all share these attributes:

  • Programmable stroke and speed;
  • Pendulum motion;
  • Dwell or holding function with full press force at bottom dead center, which enables reliable integration of other processes such as nut and stud feeding, in-die welding or in-die tapping; and
  • Time-saving setup features and tryout processes for improved safety due to the availability of full-press force at low speeds.

These attributes enable greater precision of produced parts and increased output rates. Slide motion control results in less die wear and maintenance.

The drive capabilities described above enable the following to be performed with a servo press:

  • Crank motion (sinusodial curve shape);
  • Transfer mode (allows for the adaption of the slide motion to the required feed length);
  • Drawing and fine blanking;
  • Cutting and coining; and
  • Pulse-mode-vibration forming.

Important Die-Related Factors

Attention paid to three important factors—die process, die opening and transport window—pays off in terms of part quality, process reliability, cycle-time optimization and die life.

When it comes to the die process, consider the following:

  • Adaptation of forming, drawing or cutting speeds for specific parts and process requirements;
  • Fluctuation in material and cost quality; and
  • Integration of secondary in-die processes such as welding, joining, laser operations, weld, clinch-nut and stud feeding, thread forming and adhesive bonding.

Examples of preprogrammed slide-motion curves.
Related to die openings, consider:

  • Adaption of the movement in this phase to provide clearance and accessibility for automation equipment;
  • Active in-die function—springs, sliders, part lifters, thread formers or part-turnover stations—that can be performed with high process reliability; and
  • Maximum permitted opening speed with regards to nitrogen cylinders in the die.

Regarding transport windows, consider:

  • Synchronization with automation modes and functions;
  • Clearance and accessibility in the die space.

Troubleshooting Tips

Troubleshooting servo-press issues requires an understanding of the basics. Like any other press, a servo press utilizes a spring-set brake, sized to stop the drivetrain and slide motion at any point in the 360-deg. rotation of the driving shaft, and releasing during the stroking action.

The servo drive, which controls press movement, allows the press drivetrain to move in either direction during the stroke. Upon completion of the stroking action, the press turns off and the safety brake is applied to hold the position of the slide assembly.

Watch for troublesome developments in the safety brake, servo drive, lubrication and hydraulic systems, slide assembly and slide-guiding system (gibbing), air-counterbalance system and electrical controls. This can help avoid maintenance issues.

Safety brake:

  • Sound of leaking air upon release of the brake signals that the air seals require replacement.
  • Excessive lining dust on the brake housing and press frame reveals a lining-wear issue.
  • Hot smell or excessive noise coming from the brake as the driveshaft assembly rotates warns of the need for an immediate inspection.

Check the brake travel and verify that friction plates are not rubbing as the driveshaft rotates.

Stroke configuration allows for better control of the feed window.
Servo drive:

  • Inspect (if not water-cooled) and keep clean the servo-motor cooling fan while regularly checking for overheating and listening for strange noises.
  • Change the air filter regularly.
  • Track driving hours, watching for higher-than-normal current draw on the electrical system.
  • Monitor the acceleration of the servo motor.

Lubrication and hydraulic system:

  • Check the lubrication and hydraulic reservoirs, and keep them clean and filled. Change out any inline filters on a regular basis.
  • Check for leaks and correct as needed. Keep the machine wiped down and free from excessive oil. Check all of the lines and fittings, and replace damaged components.
  • Check and benchmark system pressures as required to establish the normal conditions for a specific press and related systems.

Slide assembly and slide-guiding system (gibbing):

  • Verify parallelism between slide-face and bolster-plate surfaces, left to right under the connection assemblies and front to rear on the center of the slide. Keep the bolster plates secure to the press frame. Parallelism tolerance: 0.001-in./ft. of bed span, left to right and front to rear.
  • Check the slide-adjusting mechanism for proper function, including upper and lower-limit switch function.
  • Inspect the hydraulic overload function to determine whether overloads are holding and how often the overload pump activates during the shift.
  • Check the operational condition of pressure switches.
  • Determine if the overload reservoir is full and check for leaks.
  • Verify adequacy of oil running down the way surfaces and look for evidence of contamination or discoloration in the oil.

Air-counterbalance system:

  • Verify functionality of the air regulator.
  • Check for leaks.
  • Verify that the air-pressure setting sufficiently offsets the weight of the slide assembly and upper tooling. The counterbalance system, if properly used, will reduce drivetrain load during the stroke by lifting the weight of the slide and the upper die shoe.

Electrical controls:

  • Inspect motor-control functions and generate documentation for safety records.
  • Look for damaged or exposed wiring.
  • Check for faults.
  • Verify that all press-control features work.
  • Evaluate functionality of all pressure switches controlling press systems.

Take Extra Steps

Many additional checks and procedures should be performed as part of a press-inspection process. Companies with servo presses must develop benchmarking processes for providing critical feedback related to maximizing uptime and production. Remember, proper throughput is not limited to proper press function. Rather, metal formers must continuously strive to improve workcell best practices, addressing not only the press but the feed systems, transfer automation, stock lubrication, conveyor systems and operators. MF

Industry-Related Terms: Bed, Blanking, Center, Checks, Coining, Die, Discoloration, Draw, Drawing, Forming, Functionality, LASER, Lines, Point, Stroke, Tapping, Tolerance, Torque, Transfer
View Glossary of Metalforming Terms


See also: SIMPAC America Co. Ltd.

Technologies: Stamping Presses


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