Chris's Marion 91 Steam Shovel

[10KPete]

 
 

Pete

[cnr6400]

Hi Chris, I did a quick calculation for shear force to strip #1-72 brass screws. The shear /strip force could be 25.2 to 62.9 lb depending on hardness of the brass and the brass grade. Take these figures with a grain of salt though - they are based on the theoretical full thread area, but in real life the thread area is probably not 100% of theoretical perfect, nor will the tapped hole be.

Even at the low number 25 .2 lb per screw I doubt that the screws in your eccentrics would ever see 25.2 lb load each. The friction load on your small area valve in the engine is probably only a few pounds and the eccentric being a cam will ease the load application on the screws. They aren't loaded like say conrod bolts would be.

The 932 bronze will be considerably higher strength than the brass screws, so the screws would likely strip (if anything is going to) before the bronze strap thread. You could use replacement mild steel or stainless screws if you ever did strip the brass ones.

Just food for thought. I think the brass #1-72 screws will be fine, and if not, you could substitute mild steel or stainless ones and they would be more than enough. Don't overtighten the screws though. Snug and loctite blue will do.

[crueby]

Hi Chris, I did a quick calculation for shear force to strip #1-72 brass screws. The shear /strip force could be 25.2 to 62.9 lb depending on hardness of the brass and the brass grade. Take these figures with a grain of salt though - they are based on the theoretical full thread area, but in real life the thread area is probably not 100% of theoretical perfect, nor will the tapped hole be.

Even at the low number 25 .2 lb per screw I doubt that the screws in your eccentrics would ever see 25.2 lb load each. The friction load on your small area valve in the engine is probably only a few pounds and the eccentric being a cam will ease the load application on the screws. They aren't loaded like say conrod bolts would be.

The 932 bronze will be considerably higher strength than the brass screws, so the screws would likely strip (if anything is going to) before the bronze strap thread. You could use replacement mild steel or stainless screws if you ever did strip the brass ones.

Just food for thought. I think the brass #1-72 screws will be fine, and if not, you could substitute mild steel or stainless ones and they would be more than enough. Don't overtighten the screws though. Snug and loctite blue will do.

Great to know, thanks!!  Being so narrow, I figured the bronze for the straps would be better than brass, but was worried that the screws in brass would be too weak.

How do you calculate shear and thread strip forces for diff sizes/metals? Is there a formula set for that?
 

[crueby]

With the great Machine Screw Panic of 2018 over....   

Moving on with the follower straps - cut the blank in half, and milled the faces flat again:

Then drilled the holes for the 1-72 screws to hold the halves together, spaced wide enough to cut them apart and re-face the sides. The holes in one half are counterbored out to the clearance size, other part are to be tapped.


I've got half the holes tapped, once the other half are done I will chuck it up in the 4-jaw for boring the center hole...

[cnr6400]

Hi Chris, I calculate the area of the thread itself for a given thread (OD area - core dia area) and multiply by the yield strength of the material to get the shear strength prediction. I find the most accurate strength data comes from matweb.com but you can usually find it for various materials by googling, if you are not registered on matweb.  I use the lowest spec'd yield strength in the range rather than the ultimate tensile strength because when screws strip, the material is at yield not at UTS, and by using the lowest spec'd number the result is not optimistic, but practical. The thread area of the screw is the part under shear load, not the core of the screw, so strength before strip figures are at the lower end of the range for best certainty they will be strong enough in the worst case. If the actual screws are much stronger, so be it, no harm done.

For brass in particular, there are so many variations in the quality and hardness nowadays it is hard to predict strength, so I use figures from a well known threaded insert and hardware supplier for the range of yield strengths for the brass they use. They state the yield strength of their brass at 18,000 to 45,000 psi.  For 932 bronze (sae 660) I usually use 20,000 psi yield for calcs, and for steel 50,000 psi yield (in case the screws are Chinesium). (Good quality name brand steel screws can be up to 180,000 psi yield just FYI)

Hope this info helps.

[crueby]

Hi Chris, I calculate the area of the thread itself for a given thread (OD area - core dia area) and multiply by the yield strength of the material to get the shear strength prediction. I find the most accurate strength data comes from matweb.com but you can usually find it for various materials by googling, if you are not registered on matweb.  I use the lowest spec'd yield strength in the range rather than the ultimate tensile strength because when screws strip, the material is at yield not at UTS, and by using the lowest spec'd number the result is not optimistic, but practical. The thread area of the screw is the part under shear load, not the core of the screw, so strength before strip figures are at the lower end of the range for best certainty they will be strong enough in the worst case. If the actual screws are much stronger, so be it, no harm done.

For brass in particular, there are so many variations in the quality and hardness nowadays it is hard to predict strength, so I use figures from a well known threaded insert and hardware supplier for the range of yield strengths for the brass they use. They state the yield strength of their brass at 18,000 to 45,000 psi.  For 932 bronze (sae 660) I usually use 20,000 psi yield for calcs, and for steel 50,000 psi yield (in case the screws are Chinesium). (Good quality name brand steel screws can be up to 180,000 psi yield just FYI)

Hope this info helps.

Excellent, thanks very much! 

[crueby]

I got the rest of the holes threaded and the halves bolted together. With the pair centered up in the 4-jaw so that the seam was centered, I drilled and bored the parts out to match the eccentric cams:


Then moved the chuck over to the rotary table on the mill, and set up a slitting saw to slice off the parts - the rotary table let me rotate the part after each move of the table into the blade, so the final cut was not very deep. I was not confident of the blade staying level and not wandering, and was not sure of the finish it would leave, so I made the cuts so it left the part a little thick. It will be trimmed back to final thickness on the lathe - been wanting to try making an expanding arbor (seen it done by some of you guys on your projects), so will try making one to hold it for trimming.

[cnr6400]

Just FYI Chris, I don't like brass chips in my smoked meat on rye sandwiches, Only mentioning it as I see you found the deli slicer. 

"Morty - two sliced extra thick, no pickle, Loewensenf on both!" Oh sorry, wrong deli......

[crueby]

Just FYI Chris, I don't like brass chips in my smoked meat on rye sandwiches, Only mentioning it as I see you found the deli slicer. 

"Morty - two sliced extra thick, no pickle, Loewensenf on both!" Oh sorry, wrong deli......

Yeah, this slicer specializes in the Extra Crunchy!   

[crueby]

Okay, all the follower blanks are sliced off the main bar:


Between each slice I took the chuck back to the lathe to face the end down to the starting point for the next part, so all that needs to be done is to take the second side down to thickness. To make it easier to hold them straight, I will attempt to make up an expanding mandrel. Never made one, but have seen them on lots of other builds, should be easy (famous last words).

[steam guy willy]

Hi Chris good to see the engine coming together   

Willy

[fumopuc]

Hi Chris, nice progress, waiting for the next engine parts.

[cnr6400]

Hi Chris, the expanding mandrel will work well, but if you wanted to avoid the work of making a special one for the straps, there's another alternate method I have used in similar circumstances. If you clean the straps well and superglue them to a plywood circle held in the chuck, they can be faced with very light cuts then gently pried off with a sharp wide blade. Two sided tape can be used too. Also heard of laquer being used in a similar way to glue with a metal disk by clockmakers. I've also soft soldered parts to plates or disks for further work, then melted the parts off, but this requires solder cleanup. If the soldered faces are not visible afterward that might be fine, but if the faces show, the glue / tape/ laquer methods might be better. Just food for thought. Lots of ways to do things!

[crueby]

Hi Chris, the expanding mandrel will work well, but if you wanted to avoid the work of making a special one for the straps, there's another alternate method I have used in similar circumstances. If you clean the straps well and superglue them to a plywood circle held in the chuck, they can be faced with very light cuts then gently pried off with a sharp wide blade. Two sided tape can be used too. Also heard of laquer being used in a similar way to glue with a metal disk by clockmakers. I've also soft soldered parts to plates or disks for further work, then melted the parts off, but this requires solder cleanup. If the soldered faces are not visible afterward that might be fine, but if the faces show, the glue / tape/ laquer methods might be better. Just food for thought. Lots of ways to do things!

Excellent tips - though I think I will start with the mandrel, been wanting to try that method for a while, can go to one of your methods if it doesn't work out. The mandrel should make it easy to get them all concentric, plus I think I can use it to do the milling to shape on the outside of the straps which will put more force on than the facing will. That milling can always be done with a center hold-down if the mandrel does not work out.
 

[crueby]

Hi Chris good to see the engine coming together   

Willy

Hi Chris, nice progress, waiting for the next engine parts.

Thanks guys - its nice to be back on an actual engine again, back in the comfort zone for a while. The crosshead guide for this one will be interesting, normal open-sided one but with mounting blocks out two of the sides. Also the cylinder setup is a little different, the top/bottom caps and cylinder are not held together with studs at each end, the studs actually go all the way through and are held by the end of the crosshead guide - it saved them having to make room for nuts at the bottom end.
Don't think I have shown this image, a render of the hoist engine, this does not show the crankshaft across the middle, that is part of the slew drum gear train which sits in the middle: