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Spring center pins
- Thread starter miniwally
- Start date
I think part of the problem is that it's hard to tell which came first, the chicken or the egg. Or in this case sheared pins or loose u-bolts.
It has been my experience with the guys I've met and talked to most people don't own a torque wrench, let alone use it. I always torque my u-bolts. And I torque them regularly after installing them until they stop needing re-torqued. Usually this takes 2 or 3 tighten sessions over a few road trips/trail runs. Eventually they settle in and are at the proper torque.
So while there is nothing wrong with using higher grade center pin bolts, I still believe that the u-bolts are the key factor.
It has been my experience with the guys I've met and talked to most people don't own a torque wrench, let alone use it. I always torque my u-bolts. And I torque them regularly after installing them until they stop needing re-torqued. Usually this takes 2 or 3 tighten sessions over a few road trips/trail runs. Eventually they settle in and are at the proper torque.
So while there is nothing wrong with using higher grade center pin bolts, I still believe that the u-bolts are the key factor.
az-k5
1/2 ton status
miniwally said:
True.
Two things to add/consider.
The new Ram 2500/3500 has to center pins in each rear leaf pack. Must be a reason.
Here is the reason:
Axle torque, loading, and spring arc.
As the axle torques the spring they form a different arc (some become "S" some get flatter, ect) then they return. Loading the vehicle down flattens the springs. Each leaf has a different circumference as it is laid in the pack. As the spring flattens each edge walks a tiny little bit. Flex it a few hundred thousand times (done in a few thousand miles) and it can move around on you. That is where the center pin comes in. No matter how big of a u-bolt and how tight it is there is still some surface length change from one spring to another.
Think of how many times you have seen a leaf get crooked in a pack. Even rusted and torqued down it happens. That is the spring walking.
This is the words from an enginerding student.
A U-bolt serves in four critical related roles: • Primarily, the truck u-bolts provide the force required to clamp the leaf spring and related components firmly together. In addition to the leaf spring, these components include the top plate, axle seat, axle and bottom plate.
• The properly installed u-bolt eliminates any flexing of the leaf spring in the area between the u bolts. This is particularly critical since the hole for the centerbolt in each leaf acts as a stress concentration which would lead to rapid leaf breakage if spring flexing was not totally eliminated by the U bolt clamping force.
• By firmly clamping the spring to the axle seat the horizontal forces acting on the centerbolt are greatly reduced which in turn prevents shearing of the centerbolt.
• Proper clamping of the spring by the truck u-bolts provides the desired spring stiffness and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle.
Matt that just reinforces this.
Interesting that Dodge feels they need two pins per leaf pack.
• The properly installed u-bolt eliminates any flexing of the leaf spring in the area between the u bolts. This is particularly critical since the hole for the centerbolt in each leaf acts as a stress concentration which would lead to rapid leaf breakage if spring flexing was not totally eliminated by the U bolt clamping force.
• By firmly clamping the spring to the axle seat the horizontal forces acting on the centerbolt are greatly reduced which in turn prevents shearing of the centerbolt.
• Proper clamping of the spring by the truck u-bolts provides the desired spring stiffness and contributes to maintaining the vehicle ride height and handling characteristics as originally specified for the vehicle.
Matt that just reinforces this.
Interesting that Dodge feels they need two pins per leaf pack.
MuddinManny
Banned
miniwally said:
Physics!
Each leaf of a spring pack is forged and shot peened at a different rate which creates different arch/spring rates. Each independent leave is joined to work together, creating the desired spring or lift spring effect. The center line is the "neutral zone" where each spring leaf has a common point, or "zero arch". That neutral zone is where the center pin is used to secure the wrap together because it has almost a "zero" rate of spring. To secure a spring pack in any other location would place leverage against the pin and sheer it under force.
In contrast; ever take a military wrap apart, or see the wrap dislocate? POING! The spring pack will spread because the military wrap was forcing it to remain in a forced position. It also helps prevent the leaves from shifting sideways under the strain of flex. The more arch each leave has, the more pressure outward it will take to keep it retained, thus creating the "spring" rate.
Spring rate is simply the "memory" the pack has to return to its neutral state, which is no flex and no binding. For example, an 8" lift spring remains at 8" of lift until a load is placed on it, flexes, then when that load is removed, automatically returns to the "memory" it had, i.e., 8" lift.
Axle wrap creates a variable to the spring rate, sending opposing forces through a spring, that will work cause it to work against itself laterally and longitudily. That impacts the neutral zone where the center pin is located. That zone is meant to remain constant, and not flex side to side. That's why axle wrap is deadly to a truck because it can sheer the center pin and destroy the leaf pack.
Hopefully I've gotten that out right and not confused anyone. That isn't my intent.
Manny
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lurtch-k20-(78-90)
Registered Member
1979jimmy350 said:
I disagree, the center of grd 8 bolt is "softer" than the outside, b/c the bolts are case hardened. In removing material to reduce the dia. reduces the shear load of the bolt. The discussion of this thread. I use 3/8 SHCS w/nylock nuts, and didn't even give it a second thought as far as safety, IMO I think it is a step up from stock.
I do have to drill the spring pad to match the head of the bolt.
I guess what I wonder is how much shear stress could those leaves really be exerting on the center pin if the u-bolts are properly torqued? I concede there may still be some movement, but it's gotta be greatly dimished.
By taking some off the bolt heads, he shouldn't be weakening the bolt because he's not taking it off the shank.
By taking some off the bolt heads, he shouldn't be weakening the bolt because he's not taking it off the shank.
85mudblazin
1/2 ton status
I have snapped many center pins. They were all actual "center pins" from a spring shop. I got tired of it and made some out of a 3/8" grade 8 bolt. All breakage has been eleminated.
BUT about once a year or so I pull the springs out to replace them since they are usually pretty bent up after a years worth of wheelin. The rear ones are never really bent bad. But the fronts get pretty tweaked, I attribute alot of that to the 8* shims that I am running.
But like I said I havent "broke" one since going to the grade 8 bolts.
Also a trick to grinding the heads is to chuck up the bolt in a drill and then spin it into a moving grinding wheel, works like a champ
BUT about once a year or so I pull the springs out to replace them since they are usually pretty bent up after a years worth of wheelin. The rear ones are never really bent bad. But the fronts get pretty tweaked, I attribute alot of that to the 8* shims that I am running.
But like I said I havent "broke" one since going to the grade 8 bolts.
Also a trick to grinding the heads is to chuck up the bolt in a drill and then spin it into a moving grinding wheel, works like a champ
Just curious because I want to know if I need to be concerned, so don't take this as an attack.
Do you use a torque wrench on the u-bolts and re-torque them regularly afterwards?
For that matter what about the u-bolts, are they replaced with the centerpins? Because don't high torque fasteners stretch a certain amount to create the clamping force and then once they are stretched they loose the ability to be re-torqued properly again? Or is that a bunch on nonsense?
Do you use a torque wrench on the u-bolts and re-torque them regularly afterwards?
For that matter what about the u-bolts, are they replaced with the centerpins? Because don't high torque fasteners stretch a certain amount to create the clamping force and then once they are stretched they loose the ability to be re-torqued properly again? Or is that a bunch on nonsense?
loafer
1/2 ton status
5/16-24 Grade 8 fine thread bolt (150ksi minimum tensile strength):
- Max tensile load = 8710 lb
- max shear load (single shear) = 4900 lb
3/8-24 Grade 8 fine thread bolt (150ksi minimum tensile strength):
- Max tensile load = 13170 lb
- max shear load (single shear) = 7500 lb
Maximum clamping force is equal to the maximum tensile load. Generally you only tighten bolts to 70-90% of the ultimate tensile strength though. If you exceed the yield strength (over 70%) you have to discard the bolt and nut and replace it with new ones, which is why you generally never re-use leaf spring center bolts and U-bolts.
- Max tensile load = 8710 lb
- max shear load (single shear) = 4900 lb
3/8-24 Grade 8 fine thread bolt (150ksi minimum tensile strength):
- Max tensile load = 13170 lb
- max shear load (single shear) = 7500 lb
Maximum clamping force is equal to the maximum tensile load. Generally you only tighten bolts to 70-90% of the ultimate tensile strength though. If you exceed the yield strength (over 70%) you have to discard the bolt and nut and replace it with new ones, which is why you generally never re-use leaf spring center bolts and U-bolts.
miniwally said:We need an enginerd to come up with some shear strengths and clamping forces.
I just can't see where any strength of 3/8" bolt is going to locate be able to take much load in shear.
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loafer
1/2 ton status
The joint between the leaf pack and the spring purch is suppose to be purely a friction shear joint and the pin is only there for locating purposes...so you are pretty much right on
I also agree with using grade 8 center pins. I've broke and bent a few myself, but mostly from inadaquate assembly torqe of the U-bolt or failing to re-torque after a few days of driving
I think the spec is around 150 lb-ft for 5/8" in u-bolts.
I also agree with using grade 8 center pins. I've broke and bent a few myself, but mostly from inadaquate assembly torqe of the U-bolt or failing to re-torque after a few days of driving
I think the spec is around 150 lb-ft for 5/8" in u-bolts. SkysTheLimit said:
I swear no one even reads half the posts in a thread...
Clif notes version of this thread.
Nobody really disputes the fact that the center pin is there to hold the pack together, help with line up on assembly and takes some load during hardcore use.
To solve the issue most go to a grade 8 bolt of some sort.
Nobody knows how tight their u-bolts were right before the center pin broke when it broke.
The spring pack is to be kept from moving with the clamping force from the u-bolts. THis happens but not as well as we would like and the springs tend to "walk". The "walking" of the springs within the spring pack is what helps to bend or shear the center pin.
Loose u-bolts let the springs move independent of each other and thus increase the rate with which the center pin will bend or break.
Loose u-bolts will let the axle move around under the vehicle and shear the center pin.
All of this indicates.
Grade 8 pin to be safe.
KEEP YOUR U-BOLTS TORQUED TO SPEC.
Now let us add a twist.
In my searching I found a thread on Pirate that indicated that rounded spring perches and weak upper u-bolt plates will have an affect on center pin shearing.
I tend to agree as I have seen several instances where the pin sheared and an inspection of the spring perch revealed a very rounded perch. This would let the axle rotate if you will under the spring. This is not good.
I also have seen where a buddy had his hydro assist ram mounted to his u-bolt plate. He did this with a couple of tabs welded to an additional plate on top of the spring plate.
I swear he could break that side center pin by looking at it wrong. I think the added stress on the u-bolts let the asseembly flex under load and the weak link was the center pin. I don't know what grade pin he used. I know one was a replacement pin from a manufacturer.
Discuss
Nobody really disputes the fact that the center pin is there to hold the pack together, help with line up on assembly and takes some load during hardcore use.
To solve the issue most go to a grade 8 bolt of some sort.
Nobody knows how tight their u-bolts were right before the center pin broke when it broke.
The spring pack is to be kept from moving with the clamping force from the u-bolts. THis happens but not as well as we would like and the springs tend to "walk". The "walking" of the springs within the spring pack is what helps to bend or shear the center pin.
Loose u-bolts let the springs move independent of each other and thus increase the rate with which the center pin will bend or break.
Loose u-bolts will let the axle move around under the vehicle and shear the center pin.
All of this indicates.
Grade 8 pin to be safe.
KEEP YOUR U-BOLTS TORQUED TO SPEC.
Now let us add a twist.
In my searching I found a thread on Pirate that indicated that rounded spring perches and weak upper u-bolt plates will have an affect on center pin shearing.
I tend to agree as I have seen several instances where the pin sheared and an inspection of the spring perch revealed a very rounded perch. This would let the axle rotate if you will under the spring. This is not good.
I also have seen where a buddy had his hydro assist ram mounted to his u-bolt plate. He did this with a couple of tabs welded to an additional plate on top of the spring plate.
I swear he could break that side center pin by looking at it wrong. I think the added stress on the u-bolts let the asseembly flex under load and the weak link was the center pin. I don't know what grade pin he used. I know one was a replacement pin from a manufacturer.
Discuss
Don't know how well this will show, but the drivers side spring pad on my 60 is pretty rounded. Obviously this is how it was from the factory.
I guess I don't see how the rounded perch can contribute to a sheared pin. I could see how it could possible contribute to axle wrap, hence perches like Ruffstuff sells specifically marketed to help reduce wrap.
I know that there needs to be some radius to the corner or it'll create a stress riser in the springs. From what I understand this is why for example ORD's zero-rates have the radius edges on the top.
As for you buddy's problem, I think we often forget the unintended consequences of some of the mods to our trucks. Stuff like running poly bushings on the trans mounts and rubber on the motor mounts which for some has led to cracked bellhousings.
I guess I don't see how the rounded perch can contribute to a sheared pin. I could see how it could possible contribute to axle wrap, hence perches like Ruffstuff sells specifically marketed to help reduce wrap.
I know that there needs to be some radius to the corner or it'll create a stress riser in the springs. From what I understand this is why for example ORD's zero-rates have the radius edges on the top.
As for you buddy's problem, I think we often forget the unintended consequences of some of the mods to our trucks. Stuff like running poly bushings on the trans mounts and rubber on the motor mounts which for some has led to cracked bellhousings.
I didn't make the rounded thing very clear.
The bad perches were like half a ball type round. There was no flat spot or very little flat spot on them.
You are talking about the perches having a radius on the end and a flat spot about 3" long in the middle. The radius on the end is to reduce the stress riser deal.
The bad perches were like half a ball type round. There was no flat spot or very little flat spot on them.
You are talking about the perches having a radius on the end and a flat spot about 3" long in the middle. The radius on the end is to reduce the stress riser deal.
Okay, just got done reading a technical paper published by SAE (Society of Automotive Engineers - this is the group that probably offers the foremost in testing procedures and design requirements used in automotive design and testing).
This involves both Finite Element Analysis and laboratory testing performed by a major auto manufacturer for a leaf spring suspension system on a solid axle.
I won't bore you with the 10 pages of technical analysis, FEA pictures, and graphs, but the basic conclusion is that the centerpin does indeed provide both fore to aft and side to side strength capacity. Statically and at smaller loads (applied to the wheels) the system strictly relies upon the friction load between the springs and perch with the preload applied by the u-bolts. But at higher loads this friction force is overcome, the springs actually start "sliding" on the perch until the centerpin makes contact with the edge of the hole of the spring perch (keep in mind, we are talking very, very small measurements, like 1 mm). During this time the spring "slides" the friction force decreases because the coefficient (level of friction) is always higher in a static condition versus dynamic.
Also, as the springs twist the actual contact patch between the spring and perch change, so the contact patch can decrease and thus the total friction load decreases.
This involves both Finite Element Analysis and laboratory testing performed by a major auto manufacturer for a leaf spring suspension system on a solid axle.
I won't bore you with the 10 pages of technical analysis, FEA pictures, and graphs, but the basic conclusion is that the centerpin does indeed provide both fore to aft and side to side strength capacity. Statically and at smaller loads (applied to the wheels) the system strictly relies upon the friction load between the springs and perch with the preload applied by the u-bolts. But at higher loads this friction force is overcome, the springs actually start "sliding" on the perch until the centerpin makes contact with the edge of the hole of the spring perch (keep in mind, we are talking very, very small measurements, like 1 mm). During this time the spring "slides" the friction force decreases because the coefficient (level of friction) is always higher in a static condition versus dynamic.
Also, as the springs twist the actual contact patch between the spring and perch change, so the contact patch can decrease and thus the total friction load decreases.
