Troubleshooting Stiff Key Operation: Distinguishing Cylinder Binding from Door Sag
Engineering Overview: Diagnostic guide for identifying whether stiff key rotation is caused by internal pin binding, M5 screw over-torque, or door hinge settlement.
Technical & Engineering Specifications
| Root Causes | Door Hinge Settlement (80%) vs Cylinder Over-Torque (20%) |
|---|---|
| Diagnostic Test | Open-Door Isolation Diagnostic Test |
| Fix for Cylinder | Loosen M5 screw to 4.0 N.m; apply dry PTFE spray |
| Fix for Door Sag | Adjust 3D door hinges or file strike plate lip |
Diagnostic Flowchart for Stiff Locks
When a key is difficult to turn, diagnose the root cause with the Open Door Test:
- Open Door Test: Open the door leaf fully and turn the key.
- If the key turns smoothly with door open: The issue is door sag or strike plate misalignment, not the cylinder.
- If the key is stiff with door open: The cylinder M5 screw is over-tightened, or internal pin chambers are gummed with wrong oil.
- Remedy: Re-align strike plate or hand-loosen M5 screw by 1/4 turn.
Frequently Asked Engineering & Selection Questions
Q1: How do you tell if a stiff lock is caused by the cylinder or the door?
Turn the key with the door wide open. If it turns smoothly open but sticks when closed, the door hinges or strike plate are misaligned.