Table of contents

• Torque customer The correct tightening torque for screws
• How a tightening torque works
• Basic tightening torques
• Torque wrench
• Conclusion

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Torque customer: The right tightening torque for screws

Torque customer
The correct tightening torque for screws

Screw connections are mostly frictional connections that should be tight. To “fixed” but is known to come “away”. We explain why it is particularly important to observe the tightening torque.

Klaus Herder

09/29/2011

So that screws and nuts can exert the necessary connection holding force, they must be tightened in the thread assigned to them (either internal or external thread) with a certain tightening torque. This is the only way to guarantee that the components clamped together by screws or nuts are sufficiently tight and the screw connection does not come loose by itself, for example due to vibrations.

Example: An M6 clamping screw in the gear lever is tightened to 10 Nm, the vehicle manufacturer prescribes 85 Nm for a 16 mm front axle nut. Simply put: one “thin” Screw has less to hold and is not tightened as tightly as one “thickness” Screw that has more to hold and therefore has to be tightened more firmly.

The tightening torque is given in Nm (Newton meters). Earlier information was mkp (meter kilogram) or mkg (meter kilogram).

• 1 Nm = approx. 0.1 mkp / mkg (example 5 mkp / mkg = approx. 50 Nm).

How a tightening torque works

To put it simply: the key on the screw head has a lever one meter long. A weight of one kilogram is placed on the end of the lever (i.e. opposite the screw head) (or it is attached to it): The lever now moves downwards and moves the key with the screw in the direction “Screw shut”, until the screw is finally tightened with “1 kilogram per meter” (= 10 Nm). But because “Tighten the screw” yes is not a static process in which the force acting on the lever end can be defined in kilopond or kilogram, it is just in newtons or. “Newton meter” specify. That’s the brief explanation.

The correct tightening torque values ​​for the various assemblies are specified by the vehicle manufacturer and published in the model-specific service and repair instructions. For some machines they are also printed in the operating instructions enclosed with the vehicle. In case of doubt, the brand dealer will know.

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Example Suzuki GSF 1250 Bandit: The handlebar clamp is tightened to 23 Nm.

The correct tightening torque values ​​are particularly important for safety-relevant chassis screw connections such as axle bolts and axle nuts, the screws for the brake calliper fastening, the stanchion clamp in the lower and upper triple clamps, for the strut fastening and the bolts and nuts of the suspension lever to the rocker and frame. If treated incorrectly, the following can happen:

• Screw too loose: If screws / nuts are tightened too lightly, there is a risk that the screw connection will loosen and the part that was previously screwed down falls off.
• Screw too tight: If screws / nuts are tightened too much, their mating threads will become “soft”, they do not tear or break in the process (yet). The screw connection appears “felt solid”, but later loosens and falls off.

Notice 1: Before you start any major screwdriving work on your machine, obtain the tightening torque values ​​prescribed for your motorcycle. This is especially true for supposedly so “small and light” Work like removing and installing wheels; because the tightening torque values ​​prescribed by the manufacturer for the axles of the front and rear wheels and the brake calipers should be adhered to as precisely as possible.

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Buying two torque wrenches is not an exaggerated luxury. The small one covers z. B. the range from 5 to 40 Nm, which is particularly important for motorcycles, and the large range from 40 to 200 Nm.

Notice 2: Many screw connections must not be tightened to the final tightening torque at once. These include, for example, standpipe clamping screws in the triple clamps, collar nuts on the chainring or rear wheel screws on a cardan BMW. The manufacturer then writes in “Pre-tightening torque” which is 30 to 40 percent of the final value. Chainring collar nuts should also be tightened crosswise, step by step, like the many screws in an engine housing cover.

Below you will find standard basic tightening torques for screws that are screwed and tightened in steel or aluminum.

Basic tightening torques

Steel screw in steel thread (or steel nut on steel bolt):
Screw / nut M5 (SW * 7 or 8) 5 to 6 Nm
Screw / nut M6 (SW 8 or 10) 9 to 11 Nm
Screw / nut M8 (SW 12 or 13) 22 to 25 Nm
Screw / nut M10 (SW 14 or 17) 44 to 50 Nm
Screw / nut M12 (SW 17 or 19) 77 to 85 Nm
Screw / nut M14 (SW 19 or 21) 95 to 110 Nm
Screw / nut M16 (SW 22 or 24) 125 to 155 Nm
* SW = width across flats

Steel screw in aluminum thread (or aluminum nut on steel bolt):
Screw / nut M5 (SW 7 or 8) 3.5 to 5.0 Nm
Screw / nut M6 (SW 8 or 10) 6.0 to 7.8 Nm
Screw / nut M8 (SW 12 or 13) 14 to 22 Nm
Screw / nut M10 (SW 14 or 17) 25 to 35 Nm
Screw / nut M12 (SW 17 or 19) 45 to 60 Nm

• Normal steel bolts: have no strength designation or a “4th”-Mark on the head. The lower Nm values ​​apply to them.
• Higher quality steel screws: Have a strength designation or a “7th”-Mark on the head. The higher Nm values ​​apply to them.

Phillips and flange screws / nuts:
5 mm screw with Phillips head 4 Nm
6 mm screw with Phillips head 9 Nm
Flange screw 6 mm with SW 8 9 Nm
Flange screw / nut 6 mm with SW 10 12 Nm
Flange screw / nut 8 mm 26 Nm
Flange screw / nut 10 mm 39 Nm

• “Flange screw”: Screw with a washer firmly attached under the screw head.
• “Flange nut”: Nut with a washer firmly attached to one side of the nut.

Basics of correct assembly

• The first and most important prerequisite for correctly tightening screws and nuts is that their threads and contact surfaces are undamaged and free of dirt.
• Dirty screw threads and those with residues of sealant or screw locking agent are cleaned with a brass brush. DO NOT use a steel brush, this can damage the thread.
• Screws with rusted or damaged threads must be replaced with new parts.
• Oil the screws, nuts and washers a little before installing. Use normal engine oil for this.
• The use of liquid threadlocker is mandatory on some screw connections, e.g. B.. “Loctite” in the three qualities “easy”, “medium firm” and “high Strength”. The most common will be “medium firm” required: if high strength is required, but the screws and nuts should be loosened again without destroying the thread. Important: When using Loctite, the threads must be free of oil and grease.

Torque wrench

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Hazet specialist Heike L. with two models that are only partially suitable for motorcycles (60-320 Nm, 400-1000 Nm).

In order to be able to adhere to the specified tightening torque values ​​exactly, the use of a torque wrench is essential. A distinction is made between the rather cheap and now almost extinct indicating torque wrenches (with scale and pointer) and the releasing models (“Knuckle or crack key”). The cheap homes often offered in hardware stores are rather unsuitable for motorcycle screwdrivers, because they are oversized with a half-inch drive and a Nm range that only begins far beyond 20 Nm and often of dubious quality. A Nm scale is often missing in the discount offers, so you may be allowed to go out “ft.lbs” convert. There is also not always a reasonable operating manual or even a test certificate (DIN or ISO). Branded products with ¼ or 3/8-inch drive and a rather low Nm range are more recommended, for example from Proxxon (5-30 Nm with ¼-inch drive for 58.95 euros; among others at Polo) or Fein ( 3-15 Nm, 42.95 euros; from Louis) or even a noble professional part from top dog Hazet (5-60 Nm, 3/8-inch drive, around 320 euros).

A good torque wrench is an acquisition for life – provided you follow the essential operating tips:

1. A torque wrench is a sensitive measuring tool and should ALWAYS be treated that way.
2. A torque wrench is Not intended for loosening screws or nuts. There is only one reason for switching to left-hand rotation: there are also left-hand threads!
3. Triggering torque wrenches should always be relaxed after their last use, i.e. turned back to the lowest scale value.
4. Work calmly and evenly, do not use the torque wrench jerkily. Store the tool safely when not in use – a fall from the workbench is poison for the mechanics.
5. Give for maintenance on a regular basis. In industry, Hazet torque wrenches have to be serviced after 5,000 uses or once a year. This is certainly exaggerated for private users, but every two or three years the darling should go to customer service – which is usually cheaper than expected. Adjusting, calibrating and, if necessary, repairing costs z. B. at Hazet as a complete package including packaging and shipping for less than 50 euros.

Conclusion

Whether a screw connection is secure also depends crucially on the tightening torque. The pure feeling is not enough, especially with security-relevant connections.

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