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In industrial assembly, many believe that simply tightening each bolt to the specified torque with a high-precision tool completes a qualified joint. However, this “torque-only” mindset—ignoring tightening sequence—often creates serious hidden risks to equipment safety. No matter how accurate the tool, an incorrect tightening sequence can drastically reduce the actual clamping force generated. This condition, where torque spec is met but clamping force is insufficient, is what the industry calls “torque decay.”

To avoid the above problem, the principle “start from the center and expand symmetrically outward” must be followed. Tightening from the center first creates a stable reference bearing surface; subsequent outward symmetrical tightening allows each bolt’s clamping force to be evenly transmitted across the entire joint, preventing local over-deformation and greatly reducing torque-decay risk.
According to the bolt layout of the joint, here are the standardized tightening procedures for three common scenarios:

1. Single-row bolts (e.g. transmission-shaft cover, motor end-cover)

Step 1 (Pre-tightening):
Use a torque-controlled tool to tighten the center bolt to 50 %–60 % of the final torque. The center is the “mechanical balance point”; fixing it first forms a stable “central support surface” so the cover plate will not bend to one side when edge bolts are tightened, laying the foundation for uniform loading.

Step 2 (Symmetrical pre-tightening):
Following the “l(fā)eft-right alternating” rule, tighten the bolts outward on both sides (again to 50 %–60 %). Alternating keeps the two sides balanced and transmits preload evenly to the central reference surface.

Step 3 (Final tightening):
Repeat the same sequence and tighten every bolt to the final specified torque. This prevents stress concentration caused by fully tightening a single bolt too early and ensures the clamping force of each bolt matches its torque value.

2. Two-row bolts (e.g. gearbox housing, hydraulic valve block)

Step 1 (Pre-tightening):
Tighten the two center bolts of both rows to 50 % of target torque to establish the central datum.

Step 2 (Diagonal symmetrical tightening):
Following the “diagonal-symmetry” rule, tighten the remaining bolts outward from the middle. Bolts on the same diagonal apply forces in opposite directions, cancelling stresses and preventing housing distortion.

Step 3 (Final tightening):
Continue the same diagonal sequence and tighten all bolts to final torque, so the seal is compressed evenly across the entire face and housing cracks caused by one-side-first tightening are avoided.

3. Circular bolt pattern (e.g. flange, bearing end-cover)

Step 1 (Pre-tightening):
Tighten the bolt at 12 o’clock, then the one directly opposite at 6 o’clock, forming a “vertical force axis”. Next tighten bolts at 3 o’clock and 9 o’clock, creating a “horizontal force axis”. These two crossed axes cancel the circumferential twisting force and stop the flange from rotating relative to the seal face.

Step 2 (Cross pre-tightening):
Continue in the same cross pattern and tighten the rest of the bolts to 50 %–60 % of final torque, so preload is distributed “ring-wise” around the circle and local flange uplift that causes seal failure is avoided.

Step 3 (Final tightening):
Follow the initial cross sequence again and tighten all bolts to final torque, ensuring the flange face and gasket are pressed together with uniform compression.

Bolt tightening is never a simple “turn-to-number” operation; it is a systematic process involving mechanical balance and structural stability. The correct tightening sequence is the last critical line of defense that converts “meeting torque spec” into “reliable joint”. Only by combining sequence principles with tool accuracy, cleanliness procedures, and standard specifications can we truly guarantee long-term equipment safety and stability, preventing “big problems” caused by a “small sequence”.