Thread recommendation for cast Iron part 1

[Pravinb1234]

Hi dear friends and resp. seniors,

My topic is about to have ur valuable suggestions regarding thread type to be used in cast iron parts as per the application.

So basically I am working on design modification of cotton Ginning machine in which all the bolts were given earlier in BSW threads. We used it for more than 4 decades. But due to the shortage in availability and market trend of switching from BSW to metric bolts, we had also switched from BSW to metric threads and bolts. But as time goes on we faced some problems of getting thread failure in some casting parts like made up of FG200 grade. Then we analyzed the difference in both the threads of similar diameter and found that there is lot of variation in the thread height and pitch. We come to conclude that thread height and pitch in BSW threads is more as compared to same dimension Metric thread. Due to more thread height, the gripping between male and female thread is higher in BSW thread and due to more pitch, the quantity of material in two consecutive threads in more which results in more stripping strength to the threads.

I want your valuable suggestions after looking into the fact in above paragraph that which type of threads/bolts will be technically more beneficial to use specially in casting parts, after neglecting the issue of availability.

Thanks.

 

[tbuelna]

British Standard Whitworth (BSW) is an inch based coarse pitch thread standard using a 55deg flank angle and a controlled root/crest radius. It is actually a pretty good thread form in terms of fatigue. But is no longer used much.

The closest metric thread form I can think of would be MJ type threads, which use a controlled root radius on the external threads. But this type of thread is primarily used in aerospace. Finding commercial bolts with this thread form might be difficult. Most standard fasteners also currently use a 60deg flank angle.

 

[KENAT]

Pravin, have you actually done any calculations of the thread strength with the old BSW versus the metric thread you plan to put in?

http://www.engineersedge.com/hardware/whitworth-screw-threads-chart.htm

https://en.wikipedia.org/wiki/British_Standard_Whitworth

Perhaps rather than metric course you need to also look at metric fine, though the 60° V 55° angle may still be a issue.

https://en.wikipedia.org/wiki/ISO_metric_screw_thread

The other thing to look into is threaded inserts to reduce the stress in the cast parts. Helicoil are the obvious suggestion that springs to mind but there are other types.

Failing that you could look at custom metric thread forms or something but that would probably be unnecessary.

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[Pravinb1234]

@KENAT yes I hv done the calculations for thread engagement length but not specifically for BSW vs Metric. Also in the calculations the thread engagement length is well within limit. For an example I am having parameters of M12 vs 1/2" bolt. Pls. refer below :


Sr. No Metric Threads BSW Threads
(M12X40mm) (1/2”X1,1/2")

1 Threads per Inch = 14 TPI 12 TPI
2 Pitch = 1.75mm 2.116mm
3 Thread Height = 1.074mm 1.355mm
4 Effective Pitch Dia. = 10.863mm 10.4mm
5 Basic value = Rs 3.24 Rs. 3.93


@tbuelna, I think we can not use MJ type threads coz it's a agricultural based machine in which the bolts are going to be used, as I already told it's a cotton ginning machine. As per my knowledge MJ type threads are only used in Aeronautical industry for metric
system. But beyond my knowledge could u prefer to use that in our type of machinery application..?

 

[Pravinb1234]

@KENAT I have already gone through your provided links, thanks. Pls. see attached image for more clarity of earlier example in tabulated form. Expecting your suggestions for better type of thread system to apply in casting part of machinery having more vibrations as it's a agricultural machine.

 

[RolMec]

Dear Pravin,

just a proposal: instead of discussing thread types or so, why not present the type & nature of the failures in the first place? By understanding the design environment and the specificalities of the load / loadbearing capacity issues, there might be found another solution than to fiddle with (exotic ;-) ) threads..
Also, I'd stick to the standard bolt executions just following your own argumentation and rather would try to make that work.
Regards

Roland Heilmann
Lpz FRG

 

[Pravinb1234]

Dear RolMec,

The failure is stripping of internal threads of part which made up of FG200 material casting. After analysis we found that it is due to the lower stripping strength of the casting material. But it has also been observed that the issue is only with metric bolts not the BSW. That is because of the higher thread height and pitch in BSW as compared to Metric threads of same dimension and material which actually results in more stripping strength in casting material.

Thanks.

 

[Screwman1]

Pravin,
What is the % of thread engagement and length of engagement in your tapped holes? If you have over 1.25D of engaged length at 70% engagement you should be getting fastener failure as a failure mode if you are using grade 5, PC 8.8 bolts. Thread stripping in a cast iron nut member that is properly designed is highly uncommon.

 

[KENAT]

Pravin,

1) Looks like you need to take the change of thread form into account or else your stress analysis isn't given accurate results. Also as implied by Tbuelna part of the issue may be the radii reducing stress raisers.

2)

The other thing to look into is threaded inserts to reduce the stress in the cast parts. Helicoil are the obvious suggestion that springs to mind but there are other types.

3) Do you do any NDT or other steps to ensure you have good material where the threads are going to be put? On failed threads is it good material or is there a lot of porosity or inclusions or...

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[TheTick]

Sometimes thread problems have their source in the forming/tapping process.

Have you experimented with the taps you are using to cut the threads? Perhaps there is a fault in the particular brand of tap, or maybe they are not sharp enough. Also experiment with the process: diameter and finish of the starting hole, lubricant, and tapping speed.

 

[Tmoose]

It wasn't clear to me if the internal threads strip at assembly, or later, in service.

What are the comments of the folks creating these tapped holes?

Have any castings with stripped holes been scrapped? As suggested by others I'd Carve out a few of the stripped holes to help understand If I was tapping cast iron or swiss cheese.

Are these new castings, that presumably are duplicates of the original parts?
Does the casting have cored holes in an effort to reduce time drilling "tap drilled" holes?

If the casting quality is mechanically poor the threads can be mighty weak.

 

[Pravinb1234]

Dear all

Basically my question is about to hv the understanding about best threading system for casting parts. Will it be BSW or Metric..?

Because same thread area is getting failed in Metric system but not in BSW thread system.

Thanks.

 

[MikeHalloran]

we had also switched from BSW to metric threads and bolts.

... and BOOM! went the dynamite.

I assume the change was done as a running change in production, so now you've got a logistical mess, too. Did you change the part numbers?

The obvious solutions include going back to BSW (and making sure your dealers maintain an inventory of such at reasonable prices), or go to bigger metric bolts (adding another dimension to the logistics mess), or go to longer metric bolts (with attendant casting changes and related adjustments, probably to everything that used to clear the distal end of the bolts, or get your casting quality in order, or use thread inserts (and enlarging the bosses around the bolts and dealing with all the new clearance problems), or, or figure out the economic costs of going metric just for fun, by which time you'll be out of business anyway.

None of those obvious solutions is obviously the least awful solution, from our distant viewpoint.

An answer to Tmoose' question might be illuminative; did the threads strip at assembly, in a semi-ductile way, or did they strip in service, e.g. by fretting/corrosion?

You need to get a bunch of the stripped threads and examine them very closely.


Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

I think the lack of information about using BSW threads vs metric threads is an indication that there is no fundamental difference between.

If the switch from BSW to metric was a complete changeover then it is possible the material also changed. Even if the specification is the same, often material specifications refer to minimum properties. The older material may have been stronger.

Another potential cause is one that I heard of a while back - the company changed from one plating to another and ended up with big shop-floor problems because the lubrication ability of the plating was different and no one had changed the torque requirements. Even adding lubrication or cleaning the fasteners before installation will affect the results.

 

[Pravinb1234]

yeah, threads get striped in service after few months of working. Material is same as earlier means for BSW and Metric thread as well. Only the change was done in Thread system with same size of threads and bolts replacement.

 

[RolMec]

Dear Pravin,

quote:
The failure is stripping of internal threads of part which made up of FG200 material casting. After analysis we found that it is due to the lower stripping strength of the casting material. But it has also been observed that the issue is only with metric bolts not the BSW. That is because of the higher thread height and pitch in BSW as compared to Metric threads of same dimension and material which actually results in more stripping strength in casting material.
unquote.
So if you are not open to any other options / improvements as proposed above by others, there's but to go back to BSW. I'd not propose to use fine metric threading with a grey cast material as you use.
However, the failure mode "after some month" together with the "stripping" could imo hint to a manufacturing issue, if one assumes subsequent failure of single arcs of the thread's helix.
The material you use is clearly out of bonds as what concerns the metric fasteners standard application range. The countermeasure against stripping is increase of the engagement length. Concluding, in order to find the required engagement length you'd need to do this whole complex set of calculations yourself. You cannot use the engagement lengths as per the standard! And pls. be aware that even now (at least around here) there's still no final jugdement given as to all of the assumptions to be made. Again, the more for this anisotrope lamellar graphite low strength cast iron.
Finally, it is still not clear to me whether you compare the 40+ ys. old BSW threaded parts (which have worked so far) with the newly manufactured metric threads, or if you compare from equal starting conditions, i. e. to newly machined BSWthreaded - parts.
Regards


Roland Heilmann
Lpz FRG

 

[Pravinb1234]

Dear RolMec

I am comparing the newly manufactured metric threads to newly machined BSW threaded - parts.

 

[Artisi]

Change back to BSW fasteners, would assume they are available somewhere of manufacture your own.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Tmoose]

Plenty of cast iron parts use metric threaded fasteners quite successfully.

Do the fasteners fail pretty much all the time? Or, 1 in 20?

What are the metric and whitworth thread sizes in question ?

What is the loading these fasteners are subjected to? Is it one component, or fasteners of different sizes all over the machine.

I'd make sure there is not something hokey about the tap drill used, or something similar that is resulting in grossly weakened metric threads.

1 - test some finished metric holes by inserting the correct tap drill size in the finished tapped hole.
1A - install a few bolts by hand in the troublesome holes on a newly machined casting. The fasteners should go full depth by hand.
2 - confirm the correct metric fasteners are being used at assembly
3 - confirm and witness several of the metric fasteners are being installed at assembly. If there is a ton of prevailing torque, the bolts may not be clamping the component.
4 - drill and tap several test holes offset from the required holes. Torque bolts to failure and record the torque.

 

[Screwman1]

The fact that they are failing after several months of service is interesting. If it was just an overload failure related to the thread stripping strength, than I would expect that they would see early failures. The delayed failures indicate to me that the ultimate thread strength is not the problem- something else is going on.
I am wondering what the tap drill size is and what the condition of the tapped threads are? This delayed failure mode almost sounds like loosening followed by thread wear.
What % of the ultimate failure torque of the joints in question are the bolts being tightened to? A general rule of thumb is to tighten to 60% of the failure torque and the will get you to about 75% of yield of the weakest link. Are the parts getting tightened adequately?
As Moose said- millions and millions of metric bolts are used every week into cast iron around the world without problems, if the joint is so marginal that changing the theoretical thread height from 0.640P to 0.613P pushes it into failure modes, then there was never adequate safety margin in the first place.