Busbar / Terminal M6-M8 Threaded Hole risk

[silverstone]

Hi,

I bought these Battery Terminals from Aliexpress to use as my Battery Terminals. They claim to be rated 200A each but I plan to use them at 100A only.



I recently however talked with a friend who told me that copper is a quite soft material and won't really resist a lot torque (especially if I overtorque). Which is why we always use (either of) in industrial environment:
- Bolted connection (Bolt + Washer + Washer + Conical Washer + Nut)
- Clinch Nut embedded in the Copper Busbar (Bolt + Conical Washer + Washer screwed into the Clinch Nut)

I'm not sure if this is mainly due to the extreme amount of vibrations, thermal cycling, or probably a combination of both.

The internal thread is M6. I might get away to drill that out and use a M6 bolt + Washer + Conical Washer + Washer + Nut instead (galvanized Steel).

The external thread is M8 and that of course I cannot drill out.


Any thoughts ? I thought I remember seeing some videos on Youtube about Andy from Off Grid Garage using terminals like these. And he also used tapped & threaded busbars for his battery shelf ...

EDIT: the material is probably Brass, not pure Copper, even though the listing on Aliexpress is quite confusing about this aspect ...

 

[Steve Fractals]

Copper is the best low cost material to use, tinned copper is supposed to be a tiny bit better so don't think you'll have an issue.

 

[silverstone]

Copper is the best low cost material to use, tinned copper is supposed to be a tiny bit better so don't think you'll have an issue.

Thank you for your reply.

The question was mainly about the mechanical properties and the risk of overtorquing / undertorquing the connection when screwing into it. And the risk of the connection getting loose over time .

 

[silverstone]

Searching online they reccomend using ~ 110 inch lbs of torque for Brass fasteners, so approx. ~12.5 Nm (for 5/16''-18 bolt size) - and this is probably the mechanical limit. For M8 steel I would use normally 15 Nm for electrical connections ....

 

[silverstone]

And if I look at some PCB terminal Connector that is also M8 and Brass, such as e.g. Wurth 7461108



They reccomend only 9 Nm of torque .

 

[Q-Dog]

If I was concerned about stripping the threads I would install a stud in the flat connection then use nuts and washers to secure the lugs. I would want to keep as much surface area as possible so unless they are already stripped out I would not drill for a larger diameter stud.

For the post I would screw in an all-thread bolt from one side, then use nuts and washers to hold the lugs on the other side. That way you can hold the bolt head with a wrench and not put too much stress on the post.

 

[silverstone]

I was meaning to drill through the M6 internal thread, the one that looks like a busbar with a hole (rectangular terminal).

The M8 (the one on the circular surface) obviously I cannot drill because I won't be able to reach (it's internal to the connector, I would need to cut it in half and even then...).

So you'd put a stud on the M6 or M8 side?

I'm more afraid of the M8 because I cannot access it on the other end. The M6 I could make a blind hole if needed.

Just that we are sure what we mean by post connection and flat connection. I'm not 100% sure I follow you sorry ?.

 

[Q-Dog]

Hi,

I bought these Battery Terminals from Aliexpress to use as my Battery Terminals. They claim to be rated 200A each but I plan to use them at 100A only.

View attachment 164098

I would call the red one in your image a flat connection because it is flat, and the black one a post connection because it stands up like a fence post.

Why drill out the threads? Just screw a bolt into it.

 

[silverstone]

I would call the red one a flat connection because it is flat, and the black one a post connection because it stands up like a fence post.

Not really. It's actually the same thing, one seen from below, one seen from above.

On one side it's a flat rectangular surface like a very short busbar and it's M6 there.

On the other side it's a circle shape flat surface and it's M8.



Here is the original listing: https://www.aliexpress.com/item/100....order_list.order_list_main.43.3db11802VqLjy9

 

[MattiFin]

And if I look at some PCB terminal Connector that is also M8 and Brass, such as e.g. Wurth 7461108

View attachment 164100

They reccomend only 9 Nm of torque .

Most likely max recommendation so that you dont tear off the connector from pcb.

 

[silverstone]

Most likely max recommendation so that you dont tear off the connector from pcb.

So NOT related to stripping the M8 thread ?

But if that torque is listed there, it means it's also "OK" for the power connection (so that it doesn't get loose over time), right ?

 

[Q-Dog]

Not really. It's actually the same thing, one seen from below, one seen from above.

On one side it's a flat rectangular surface like a very short busbar and it's M6 there.

On the other side it's a circle shape flat surface and it's M8.

View attachment 164113

Here is the original listing: https://www.aliexpress.com/item/100....order_list.order_list_main.43.3db11802VqLjy9

I understand they are the same thing, but the red one has the flat side up and the black one has the post side up. Don't make this harder than it really is.

 

[MattiFin]

So NOT related to stripping the M8 thread ?

But if that torque is listed there, it means it's also "OK" for the power connection (so that it doesn't get loose over time), right ?

I wouldn't count on M8 torqued to only 9Nm to hold over time if there is vibration or temperature cycles.

 

[MattiFin]

Hi,

I bought these Battery Terminals from Aliexpress to use as my Battery Terminals. They claim to be rated 200A each but I plan to use them at 100A only.

View attachment 164098

I recently however talked with a friend who told me that copper is a quite soft material and won't really resist a lot torque (especially if I overtorque). Which is why we always use (either of) in industrial environment:
- Bolted connection (Bolt + Washer + Washer + Conical Washer + Nut)
- Clinch Nut embedded in the Copper Busbar (Bolt + Conical Washer + Washer screwed into the Clinch Nut)

Bolted connection is better in many ways but threaded hole in copper or brass can be usable option also if these are not used in some extreme environment.
There is also easily 10-fold difference between yield strength or hardness of annealed electrolytic copper and 3/4 hard brass, brass being similar to ordinary structural steels.

Long thread engagement length helps in softer materials. Guessing from the foto you can use reasonably long bolt in the M8 thread.

Thread engagement length calculator

Calculate the theoretical minimum required thread engagement length for blind hole and through hole applications. Try out our free online calculator now!

www.bossard.com

M8 thread in softest CuZn37 brass needs 13mm thread engagement to archieve same strength as normal grade 8.8 bolt.

 

[silverstone]

I wouldn't count on M8 torqued to only 9Nm to hold over time if there is vibration or temperature cycles.

That's also my concern. But also the other value of 12.5 Nm I came up with based on some research for Brass fasteners seems quite low for a M8.

Not sure if the 12.5 Nm is a "target" for electrical connections in Brass or rather the limit value above which the thread starts to get destroyed.

Mechanical "limit" for steel would be ~ 25 Nm depending on the strength class. And we normally would use 15 Nm for electrical connections where i work (again with steel fasteners).

 

[silverstone]

Bolted connection is better in many ways but threaded hole in copper or brass can be usable option also if these are not used in some extreme environment.
There is also easily 10-fold difference between yield strength or hardness of annealed electrolytic copper and 3/4 hard brass, brass being similar to ordinary structural steels.

Long thread engagement length helps in softer materials. Guessing from the foto you can use reasonably long bolt in the M8 thread.

Thread engagement length calculator

Calculate the theoretical minimum required thread engagement length for blind hole and through hole applications. Try out our free online calculator now!

www.bossard.com

M8 thread in softest CuZn37 brass needs 13mm thread engagement to archieve same strength as normal grade 8.8 bolt.

I actually did see another "Thread engagement" article here:

Thread Engagement Chart

In this article we cover thread engagement, a crucial metric while selecting fasteners. Understanding how to calculate and choose the appropriate thread engagement can help ensure the strength and effectiveness of the fastener.

tameson.com

There they reccomended roughly:
- 1.0-1.5 x nominal diameter of engament for steel
- 1.5-2.0 x nominal diameter of engagement for brass
- 2.0-2.5 x nominal diameter of engagement for aluminum

The one you linked from Bossard is way more complete though. Thanks .

Brass should be H62 so probably the Brass you referred to.

I'm not sure if I have 13mm though. That would be 1.625 x nominal diameter (M8 = 8mm I assume), so in the "range" as the other article I saw.

 

[MattiFin]

That's also my concern. But also the other value of 12.5 Nm I came up with based on some research for Brass fasteners seems quite low for a M8.

12.5 Nm sound about right for brass bolt or normal nut(0.8xD thread engagement length). 2xD female thread can take considerably more.

 

[silverstone]

12.5 Nm sound about right for brass bolt or normal nut(0.8xD thread engagement length). 2xD female thread can take considerably more.

Maybe silly question but the longer the engagement, the more resistance, thus the "sooner" I reach the target torque, the lower the "stress" on each turn of thread ?

I always thought torque values were "standardized", but maybe that's because I always saw standard nuts being used (thus fixed number of turns and fixed torque for e.g. M8).

 

[silverstone]

12.5 Nm sound about right for brass bolt or normal nut(0.8xD thread engagement length). 2xD female thread can take considerably more.

I just checked. The external (flat round surface) M8 thread engagement is at least 20mm. I could get a M8x20 bolt ALL the way in (without cable lug, washer, etc).

On the other hand the internal (flat post) M6 thread engagement is <= 8mm (thickness of the small "busbar").

 

[silverstone]

As it looks now for the "flat" part I plan to leave it as it is and just screw a bolt with washer+conical washer into it and call it a day. M8 x 20mm should be far sufficient ...

For the "post" part M6 x 8mm that seems a bit short.
The options are:
- Use as it is (not sure if enough strength / torque withstand of the Brass busbar)
- Use the threaded hole and add a nut+washer on the other side. Conical washer + washer on the BOLT side. Not sure if there are issues with this approach, e.g. only the Brass busbar takes most of the torque, or Brass busbar & nut "fight" each other instead of tightening the bolt
- Drill the thread out and use Bolt + Washer + Washer + Conical washer + Nut as "standard"

Any last thoughts ?

If everything goes well I'll have a go at it tomorrow .