Crawling Phenomenon in Press Machines — Causes, Impacts, and Solutions

In the metal processing industry, the press machine is one of the most commonly used types of equipment. Whether for sheet metal stamping, part forming, or precision mold processing, presses play an extremely important role. However, during press operation, a special phenomenon may occur, known as the crawling phenomenon (also called stick-slip motion). This phenomenon mostly occurs at low speeds or during inching operations. If not resolved, it not only affects machining accuracy but may also shorten the service life of molds and equipment.

This article provides a comprehensive overview of the crawling phenomenon from four aspects: definition, causes, impacts, and solutions.

1. What is the Crawling Phenomenon?

“Crawling” refers to the situation where the slide or worktable of a press cannot maintain smooth and continuous motion at low speeds, but instead moves forward intermittently and in a jerky manner.

This movement resembles the creeping motion of an insect, which is why it is vividly described as the “crawling phenomenon.”

Example: During die testing or fine adjustment, the operator lightly jogs the slide, expecting it to descend smoothly. In reality, the slide may pause momentarily, then suddenly jump down a small distance, stop again, and continue in the same stop–go pattern. This irregular motion is a typical sign of crawling.

2. Causes of the Crawling Phenomenon

Crawling is not caused by a single factor but rather by the combined influence of friction, lubrication, machining accuracy, and loading conditions. The main causes include:

(1) Nonlinear Characteristics of Friction

In mechanical transmission, friction is divided into “static friction” and “dynamic friction.” Normally:

• The coefficient of static friction > the coefficient of dynamic friction.

This means that greater force is required to overcome static friction to start motion. Once movement begins, friction suddenly decreases, causing a sudden acceleration. This “stop–move–stop” pattern produces crawling.

(2) Poor Lubrication

If the lubrication film is insufficient or damaged, metal-to-metal contact occurs between friction surfaces. This results in fluctuating resistance, making motion unstable, especially at low speeds, under heavy loads, or when oil passages are blocked.

(3) Insufficient Accuracy of Guideways or Slides

The slide and column guideways of the press require high fit accuracy. Poor surface finish, geometric deviations, or local high points can lead to uneven distribution of friction, increasing the likelihood of crawling.

(4) Transmission System Clearance and Low Rigidity

If the crankshaft, connecting rod, gears, or other transmission components have looseness or backlash, power delivery at low speeds becomes discontinuous, causing sticking and jerking that intensify crawling.

(5) Uneven or Insufficient Load

When running without load or under light loads, the driving force is close to the frictional resistance, making crawling more likely. By contrast, under stable loads, motion tends to remain smoother.

3. Impacts of Crawling

Although crawling may appear minor, it has significant negative effects on long-term performance and machining quality:

1. Reduced Machining Accuracy
   Jerky slide motion directly increases dimensional and geometrical errors in the workpiece, especially in high-precision operations such as fine blanking and deep drawing.

2. Worsened Surface Quality
   Workpiece surfaces may show streaks, ripples, or irregular impressions, lowering both appearance and performance.

3. Shortened Mold Service Life
   Due to slide vibration, molds experience uneven impacts during engagement, which can cause chipping, cracking, or premature failure.

4. Unstable Equipment Operation
   During die testing or inching adjustments, crawling reduces control precision and may even create safety hazards.

4. Solutions to the Crawling Phenomenon

To mitigate crawling, improvements must be made in lubrication, accuracy, structural design, and control systems. Common solutions include:

(1) Improve Lubrication Conditions

• Select lubricants with proper viscosity and performance to enhance oil film strength.

• Regularly inspect oil passages and maintain automatic lubrication systems.

• Apply wear-resistant coatings on friction surfaces of guideways and slides to reduce friction.

(2) Enhance Manufacturing and Assembly Accuracy

• Optimize machining processes of guideways and slides, ensuring higher surface finish.

• Control fit clearances so that sliding parts are neither too loose nor too tight.

• Strengthen inspection and alignment during assembly to avoid eccentric loading.

(3) Optimize Transmission Structure and Rigidity

• Minimize backlash in transmission systems by using high-precision gears and bearings.

• Increase structural rigidity to prevent micro-deformation at low speeds.

• Use hydraulic balancing devices or add dampers if necessary.

(4) Adjust Load and Control Methods

• Apply preload during die testing or no-load running to avoid crawling caused by light load.

• For hydraulic or servo presses, adopt closed-loop control to ensure low-speed stability.

• Use advanced electronic control systems to achieve precise control of speed and position.

5. Conclusion

The crawling phenomenon in presses essentially results from mismatches between friction characteristics and transmission performance, leading to uneven low-speed motion. It not only reduces machining accuracy but also shortens mold and equipment lifespan, and even poses safety risks.

By improving lubrication, enhancing precision, optimizing structures, and upgrading control systems, crawling can be effectively reduced or eliminated, ensuring smooth low-speed and inching operations. This, in turn, guarantees the high accuracy and high efficiency of stamping production.

📌 Practical Tip:
For ordinary factory users, regular maintenance and proper operation are the keys to preventing crawling. If vibration, unstable inching, or abnormal machining accuracy occurs, check the lubrication system, guideway precision, and transmission clearance promptly. Contact the equipment manufacturer for professional maintenance if necessary.