How much torque should I use on my endbell screws?
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We are often asked to provide specific torque values for screws that secure endcap assemblies to threaded plastic instrument housings.
The endcap is removed for access when, for instance, changing the battery.
POM (Polyoxymethylene, also widely known as Acetal plastic or by the DuPont Delrin trade name) is an engineering thermoplastic with great properties for underwater use. These properties make POM an excellent choice for use in our instruments, but threads may be susceptible to stripping if tightened improperly.
Providing a specific torque value may result in a range of screw preloads and thread deformation, mainly dependent on the amount of friction in the system.
A high portion of applied torque transferred as preload may result in plastic deformation and possible stripping of the plastic treads. Recommended torque values should be used with caution and in conjunction with the instructions.
The same procedure applies to removing/replacing the transducer head. However, heads should not be removed by the user unless required and only after contacting Nortek Support.
This description will show you how to torque your endcap correctly.
The acting pressure of water during a deployment is working in our favor by squeezing the housing assembly together. Thus, fasteners coming loose during a deployment rarely results in a leak. Loose screws ahead of a deployment, however, may cause water ingress before pressure properly seats the O-rings.
Most water leak damage can be traced back to O-rings in poor condition, damaged sealing surfaces or contaminants on O-rings and/or sealing surfaces.
Therefore, before closing and torquing your instrument:
- Check thoroughly the inside sealing surface of the canister, where the outside of the O-ring will seal against. There should be no scratches or burrs. Even a straw of hair may cause water ingress past the O-rings over time under the right circumstances.
- Replace and grease the O-rings. This is cheap insurance.
Reusing O-rings is not something we recommend unless you know they are in good condition. - Check that everything is clean.
- Be careful with wires when closing the instrument. They may get trapped and squeezed, preventing proper mating.
- When re-inserting the screws, make sure they enter the threads correctly. They should enter with ease. If you need to use force, stop and re-enter the screw. Turning the screw slowly counterclockwise until you feel/hear it "falls" down in the first winding of the thread is a good way to make sure you enter the threads correctly.
- Never re-use spring washers.
In combination with, or in lieu of a torque wrench, use the following procedure:
- Screws and threads should be clean and dry. The recommended torque values are based on dry threads.
- Tighten screws in a star-pattern with slight increments in torque until the endcap is seated against its flange.
NEXT: - With a slow and controlled motion, proceed by applying torque up to the specified value.
It is of great importance to turn slowly and evenly in order to give the plastic time to deform. If the wrench is turned too quickly, a “false” peak torque reading may be reached before the plastic has time to relax. Approximately 5 degrees/second is a suitable rate.
OR:
If a torque wrench is unavailable, continue tightening in a star-pattern. When the spring washer is compressed, make an additional fraction of a turn according to the table below.
IN ANY CASE:
If the specified torque is not achieved at specified rotation past spring washer compression, STOP APPLYING TORQUE.
Both methods work equally well. Use a torque wrench for the correct torque range.
The tools we provide with the instrument are small and ideal to tighten endcaps as they do not provide excess leverage. If you use larger tools remember they achieve a high torque very easily, so be careful!
Screw size |
Torque |
Turn past compression |
#6-32 UNC |
70 N·cm / 6 In·lb |
Approx. 1/4 |
#8-32 UNC |
160 N·cm / 14 In·lb |
Approx. 1/4 |
#10-24 UNC |
230 N·cm / 20 In·lb |
Approx. 1/8 |
¼”-20 UNC |
500 N·cm / 45 In·lb |
Approx. 1/6 |
#6-32 UNC used in: Aquadopp, Signature 1000, DVL 1000
#8-32 UNC used in: DVL 500 (Transducer head only)
#10-24 UNC used in: Signature 500, Signature 250
¼”-20 used in: Signature 100, Signature 55, Signature battery canister
And as always; if you need further help please send us an email to support@nortekgroup.com with a detailed explanation and we will help you as quickly as possible.
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