Why Servo Press Fitting Relies on the Force–Displacement Curve

Conventional press fitting only asks "did it reach the position?"; servo press fitting asks "how did it get there?". The force–displacement curve is what separates servo pressing from ordinary pneumatic pressing, and it is the key data set for assembly traceability.

1. What the curve contains

During the stroke, a servo press samples force and displacement simultaneously at high frequency, producing a real-time curve. A normal interference fit shows a smooth rise reaching the target force at the target position. Abnormal shapes map directly to specific defects: excessive force at the start may mean the part was not seated; a sudden mid-stroke drop may mean a cracked part or jumped teeth; insufficient end force may mean too little interference or a failed spring; force beyond the limit may mean excessive interference or a foreign object.

2. How to set judgement windows

The common engineering practice is to define several windows on the curve: the contact-point displacement range, force upper/lower limits at specific displacements, and end-point force and displacement ranges. Windows come from statistics of measured good parts, not guesswork. Too strict and good parts are rejected; too loose and defects slip through — they must be calibrated repeatedly against a batch of trial assemblies.

3. What an NG part tells you

The other value of curve judgement is that it "gives a reason for rejection". Two NGs with different signatures have completely different root causes: insufficient force points to incoming part dimensions, excess displacement points to feeding or fixturing. Classifying NG curves by shape points directly at material, fixture or pressing parameters — something a simple pass/fail signal can never provide.

4. Points to confirm before you buy

Curve data must be saved per part and bound to the product barcode to satisfy downstream traceability requirements; sampling rate and sensor accuracy must match the defect signatures — detecting fine cracks needs a high sampling rate, while detecting a missing part is far less demanding. Confirm these at equipment selection; retrofitting traceability afterwards is expensive.

Send us your pressing parts and process requirements: our servo press stations support real-time force–displacement judgement and data traceability, and judgement windows can be verified with sample parts first.