Alan,
I know stainless has less capacity, however since the applicationis handlebar risers, and the stock M12 (handlebar holder - lower) riser bolt is spec'd to 29 ft/lbs torque (page 2-24), I wanted to ask if you're saying the 18-8 stainless socket head screw is under-capacity for the application, or if you're saying its simpy better to use all the bolt strength you can get for the handlebar riser bolts. The 1/2" 18-8 stainlessscrew torque value is 750 pounds, and the application calls for 29 pounds. I'm asking because at times the fasteneris nowhere near being the weak link when applied in one case,but is a weak link when other factors are considered.
18-8 stainless is known to not have galvanic actionwith aluminum so no 'dissimilar metals' issue exists. This type bolt has a very deep and solid allen head socket and its rare to damage a socket. But it costs more, maybe its just a waste of money?
Just to save you time here's somecharts stolen from the web at random Iposted only unified right coarse and fine.
Mechanical Properties of Stainless Steel Socket Cap Screws
Nominal Size
Tensile Strength (lbs.,min.)
Yield Strength (lbs.,min.)
Body Section
Tightening Torque (In.-Lbs.)
UNRC
UNRF
UNRC
UNRF
Single Shear Strength (lbs.,min.)
UNRC
UNRF
2
295
-
185
-
260
3.8
-
4
480
-
240
-
350
6.0
-
6
725
-
363
-
375
15.0
-
8
1120
-
560
-
670
28.0
-
10
1400
1600
701
800
950
40.0
46.0
1/4
2550
2910
1273
1455
2200
95.0
109.0
5/16
4200
4645
2100
2320
3450
170.0
188.0
3/8
6100
7025
3100
3510
4970
301.0
341.0
1/2
11350
-
5675
-
8840
750.0
-
Mechanical Properties of Alloy Steel Socket Cap Screws
Nominal Size
Tensile Strength (lbs.,min.)
Yield Strength (lbs.,min.)
Body Section
Tightening Torque (In.-Lbs.)
UNRC
UNRF
UNRC
UNRF
Single Shear Strength (lbs.,min.)
UNRC
UNRF
0
-
320
-
290
305
-
2.6
1
-
500
-
450
450
-
4.8
2
665
-
600
-
625
7.5
-
3
875
-
790
-
830
11.0
-
4
1090
-
975
-
1060
16.0
-
5
1430
-
1290
-
1325
24.0
-
6
1640
1825
1470
1645
1615
30.0
34.0
8
2520
2650
2270
2385
2280
55.0
58.0
10
3150
3600
2835
3240
3060
79.0
90.0
1/4
5725
6550
5150
5900
5295
200.0
230.0
5/16
9430
10440
8490
9395
8285
415.0
460.0
3/8
13950
15805
12555
14225
11910
740.0
845.0
7/16
19135
21365
17220
19230
16200
1190.0
1305.0
1/2
25540
28780
22990
25905
21175
1800.0
2065.0
5/8
38400
43500
34550
39150
31300
3400.0
3800.0
3/4
56750
-
51100
-
45050
6000.0
-
7/8
78500
-
70700
-
61350
8250.0
-
1
103000
-
92700
-
80100
12500.0
-
1-1/4
164700
-
148250
-
125100
25000.0
-
1-1/2
238800
-
215950
-
180200
43500.0
-
I stole this from the web too, in case helpful. Its admittedly generic but applies to this case with respect toselecting hardware.
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Sizing and Strength: How big a screw is needed?
At first thought, sizing a screw for a given load would seem to be a simple matter. If you need to hold 100 lbs, find a screw that can hold 100 lbs before it yields...
But things are not so simple. If a screw can withstand 100 lbs of force before yielding, it is recommended for a number of reasons (discussed in the next section) that it be tightened to about 80 lbs of tension / clamping force
just for installation alone. Does that mean that it can only withstand another 20 lbs of external load before it yields? Why would we tighten a screw so much if we're using up the majority of its strength just to hold it in place? It turns out that only a portion of the external load is seen by the bolt, a rough estimate is about 1/3, but this depends on lots of things.
Here's a rough guide for picking a screw or bolt for a given load:
Start off with the load that needs to be held in tension, call this F. If you have a shear (sideways) load, you should design so that friction or dowell pins will bear the load and not the bolt, but if this isn't an option note that shear strength is 60% of tensile strength in many steels.
We'll use a safety factor of 2.5, so the design load is now 2.5F. Now we need to select a screw with enough strength so that it can withstand the combined external load and pre-load from tightening. Assuming that 80% of the bolt's proof strength is being used up in preload, that leaves 20% to handle 1/3 of the external load. Or in other words,
we're looking for a bolt where 60% of its proof strength is greater than the load.
Let's try an example: What size grade 2 bolt is necessary to hold 100 lbs? The proof strength of Grade 2 bolts between .24 and .75 inches is 55 ksi (thousand pounds per square inch), and 60% of this is 34.2 ksi. So, we're looking for a bolt with a tensile area greater than our load (2.5*100 lbs) divided by 33 ksi, or .0076 square inches. A #6 UNC should work. For perspective the diameter of a #6 screw is .138", (1/8 = .125"). If this seems small, keep in mind that the ultimate strength (breaking strength) of a Grade 2 bolt is 74 ksi, so a #6 screw could theoretically hold 672 lbs in pure tension. If you're wondering why bolts you see in cars and weight machines are so large, it's partly to gaurd against loosening and fatigue failure in addition to safety factors.
What about changing loads? According to this Unbrako
whitepaper on the Fastener Act, over 85% of failures are due to fatigue and not a simple overloading situation. Think about breaking a paper clip, which is easier: bending it back and forth or out-right pulling it apart? If you have an oscillating load and want a joint to last forever, the best advice we can offer is to multiply the anticipated load by 10 or more, and even this may not be sufficient. Steel can handle about half of its ultimate strength in an alternating load, but add in the pre-load stress and something called a "Stress Concentration Factor" due to the threads and the problem gets more complicated quickly.
Here's a good explanation of these effects along with a lot of other great screw information.
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Alan - Thanks in Advance!