The Samples Were Rock Solid, but the Production Chairs Came Loose Within a Month
Screw Torque and Fastening Control Guide - Why Screws Loosen and Strip in Mass Production, Tightening vs. Breakaway vs. Failure Torque, Setting Electric Screwdriver Torque and Checking It with a Torque Tester, Threadlocker, Washers and Other Anti-Loosening Methods, Spec and Work Instruction Items, Stage-by-Stage Checks and a Checklist

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A Korean brand selling folding camping chairs had 3,000 aluminum chairs made at an outdoor furniture factory in Yongkang, Zhejiang. The three samples had been sat on in the office for weeks without a wobble, and the pre-shipment inspection passed on appearance, dimensions and function. But a month after launch, reviews started coming in: "The leg joint rattles," and "I sat down and it tilted to one side." Over three months, returns and replacement-part requests reached about 210 cases, 7% of sales.

Checking the returned chairs showed that most of the M5 bolts joining the legs to the frame had backed out by half a turn to a full turn. When asked, the factory explained that the samples had been tightened by an engineer with a torque wrench and threadlocker, but on the production line workers drove the bolts with electric screwdrivers and skipped the threadlocker. The torque setting on each screwdriver was different from station to station, and no document defined what it should be. The pre-shipment inspection only checked by hand that the screws were tight.

Loose screws are almost invisible at the time of shipment. They show up only after customers have used the product for a few weeks under vibration and load. This guide covers why screws loosen or strip in mass production, what each type of torque means, how to control fastening tools and torque, how to prevent loosening, and what to check in your spec sheet and inspections.

"Tighten firmly" is not a specification. Production units hold up like the samples only when every fastener has a torque value, a tolerance and an anti-loosening method defined in numbers and part names, and you confirm with records that the line actually follows them.

1. Why Screws Loosen and Strip in Mass Production

Samples are assembled slowly by one or two engineers, while production units are assembled by many workers within a fixed takt time. Most fastening defects come from that gap. These are the causes we see most often.

• No torque setting, or a different one at every station: Workers set the clutch on the electric screwdriver (电批) by feel. Too low and the screws loosen; too high and the threads strip or plastic bosses crack.
• Anti-loosening step dropped: The samples used threadlocker (螺纹胶) or spring washers, but production skipped them. Nothing looks different from the outside.
• Screw spec changes: Screws with a different length, material, property class or finish get mixed in, or are swapped for cheaper ones.
• Tightening sequence and angle: If the sequence isn't followed on multi-screw joints, or a screw goes in at an angle, one side ends up tight and the rest stay under-tightened.
• Variation in the mating parts: If boss hole diameters or tapped depths differ between cavities, the same torque gives different results. Dimensional control was covered in Episode 134.

The operating environment matters too. Products under vibration and repeated loads, such as bicycles, strollers and camping chairs, or products that get shaken for a long time in transit, will loosen quickly if the clamping force falls even slightly short.

2. Tightening, Breakaway and Failure Torque: What's the Difference

Agreeing on terms with the factory before you talk torque prevents misunderstandings. Torque is usually expressed in N·m (牛·米) or kgf·cm; 1 N·m is about 10.2 kgf·cm.

For plastic bosses, set torque between drive and strip torque

When self-tapping screws go into plastic, the tightening torque must be comfortably above the drive torque and comfortably below the strip torque. For example, if drive torque measures 0.3 N·m and strip torque 1.2 N·m, you would set it around 0.6–0.7 N·m. These values change when the mold or resin changes, so measure 5–10 parts per cavity from the final mold before deciding. Resin grade control is covered in Episode 125.

For metal bolts, start from property class and clamp load

For metal-to-metal bolted joints, start from the recommended torque table for the bolt's property class (for example 8.8, or stainless A2-70) and thread size. Even with the same M5 bolt, the clamping force (preload) you actually get at a given torque varies with plating, lubrication and threadlocker. That is why the torque value has to be confirmed by testing with the actual production parts and the actual screws.

3. Fastening Tools and Torque Control

Once the torque value is set, you need to control whether the line actually delivers it. Here are the tools commonly used in Chinese assembly factories and what to check for each.

Electric screwdrivers vary slightly in output even within the same model, and the clutch wears with use. Post the setting at each station and put a procedure in the work instructions to measure each screwdriver three times on a torque tester before every shift and record the results. A seal sticker on the adjustment ring also cuts down on workers turning it on their own.

4. How to Prevent Loosening

Even with accurate torque, screws can gradually loosen under vibration and repeated loads. Choose a method that suits the product structure and whether it needs to be taken apart, and write the chosen method into the spec sheet down to the part name.

On plastic parts, some threadlocker ingredients can crack PC and ABS. For plastic joints, use a plastic-safe threadlocker, or consider pre-coated screws or washers. Require prior approval for any change to anti-loosening parts. Change control is covered in Episode 119.

5. What to Put in the Spec Sheet and Work Instructions

A line like "tighten screws firmly" gets interpreted differently at every factory. For each joint, put the items below in a table and attach it to the spec sheet or BOM. BOM writing is covered in Episode 59.

If a product uses several screw types, keep similar-length screws from getting mixed up by giving each station only one screw type, or by using kitting trays that supply a counted number of screws. Leftover screws in the tray also reveal a missed fastener immediately.

6. Checks at Each Stage

Sample stage

When samples arrive, measure the breakaway torque of key joints with a torque screwdriver or torque wrench, and take them apart to confirm the screws and anti-loosening parts match the spec. For plastic bosses, ask the factory for drive and strip torque test results. For products that see a lot of vibration, measure breakaway torque again after a vibration or transit test. Transit testing is covered in Episode 127.

First article and production stage

On the first day of production, check each station's screwdriver setting, the torque tester check records and how threadlocker is being applied. Also confirm that the torque values and sequence agreed in the pre-production meeting made it into the work instructions. First article checks are covered in Episode 123.

Pre-shipment inspection stage

A standard AQL inspection usually only checks whether screws are missing and does not measure torque. Add breakaway torque measurement on key joints (for example, 5 pieces per lot per joint), torque marking checks and threadlocker checks to the inspection checklist. Pre-shipment inspection is covered in Episode 100.

7. Sample Request Wording for the Factory

Putting the wording below in your purchase order or spec sheet takes fastening conditions out of the factory's discretion and makes it easier to assign responsibility if loosening claims come in.

"所有关键锁付部位须按我方确认的《锁付规格表》执行，包括螺丝规格、锁付扭力及公差、锁付顺序和防松方式。电批须按工位设定扭力并贴标识，每班开线前须用扭力测试仪点检并保存记录，超出范围须停用调整。螺丝、螺纹胶、垫片如需变更，须事先取得我方书面确认。出货检验须对关键锁付部位抽测松脱扭力（每批每部位至少5个），并提供测试记录。"
(All key joints must follow the Fastening Specification Table we have approved, including screw specs, tightening torque and tolerance, tightening sequence and anti-loosening method. Electric screwdrivers must be set to the specified torque for each station and labeled, checked with a torque tester before each shift with records kept, and taken out of use and adjusted if out of range. Any change to screws, threadlocker or washers requires our prior written approval. Outgoing inspection must sample breakaway torque on key joints (at least 5 pieces per joint per lot) and provide the test records.)

8. Three Real Cases

These are anonymized versions of patterns we often see in consultations. Amounts and figures are examples.

Case 1 — Aluminum camping chairs

This is the case from the start of the article. Working with the factory, threadlocker went back on the leg-joint bolts, and tightening torque was set at 5.0 N·m ±10% and posted at each station. Screwdrivers were checked on a torque tester before each shift, and torque marking was added after tightening. On the next run, breakaway torque was measured directly during pre-shipment inspection. The torque tester and threadlocker cost less than 10,000 yuan, while the 210 replacement-part cases already shipped and the drop in star ratings cost far more.

Case 2 — Small humidifier housing

A small humidifier with a PC+ABS housing held together by six self-tapping screws drew a string of claims that housing corners had cracked during delivery. It turned out some stations had turned screwdriver torque up to maximum to hit output targets, leaving fine cracks in the bosses that split open under transit vibration. After drive and strip torque were re-measured per cavity, tightening torque was lowered to 0.8 N·m and the adjustment rings were sealed, and the same claims stopped.

Case 3 — Long screws in the wrong holes

An importer of children's wooden toy shelves got a complaint that screw tips were poking through the back panel. The station used 12 mm and 16 mm screws side by side, and 16 mm screws had gone into holes meant for the short ones. Because it was a children's product and could become a safety issue, every shipped unit was re-inspected, and each station was switched to a single screw type with counted kitting trays. Safety requirements for children's products are covered in Episode 65.

9. Common Mistakes

• Writing only "tighten firmly" in the spec without a numeric torque value
• Not writing the threadlocker or washers used on the samples into the spec
• Never checking screwdriver settings and check records
• Setting plastic boss torque without measuring drive and strip torque
• Checking screws only by hand at pre-shipment inspection without measuring breakaway torque
• Letting one station use similar-length screws side by side

10. Screw Torque Quality Checklist

Spec and contract stage

• Screw spec, tightening torque and tolerance are set for every key joint
• Tightening sequence and anti-loosening method (with part name) are written down
• Plastic boss torque is based on measured drive and strip torque
• Prior approval is required for changes to screws, threadlocker or washers

Sample and first article stage

• Breakaway torque on the samples was measured directly
• Samples were taken apart to confirm screws and anti-loosening parts match the spec
• Screwdriver settings per station and torque tester records were checked

Production and shipment stage

• Torque tester check records exist for every shift
• Each station uses one screw type or counted kitting trays
• Pre-shipment inspection includes breakaway torque measurement and torque marking checks

Wrapping Up - Turn "Firmly" into a Number

Loose screws show up only weeks after the product reaches customers, and when they do, reviews and replacement-part requests arrive all at once. Setting a numeric torque for each joint and having the line keep check records costs very little.

There is something you can do right now. Open the spec sheet for a product you sell today and check whether the key joints list a torque value and anti-loosening method in numbers and part names. If they don't, make a fastening specification table and send it to the factory before your next order.

StartMade supports you on the ground with fastening specification tables, requesting torque tests for plastic bosses, checking screwdriver settings and records at each station, and pre-shipment inspections that measure breakaway torque. If you already have loosening claims, we start with the returned units; if you are preparing a new product, we start with the fastening specification table.