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Coach bolts or coach screws?
Apart from reasons of practicality, is there any reason for not using
coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg |
Coach bolts or coach screws?
dg wrote:
Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg Personally I prefer bolts,, but screws are pretty damned good. Screws CAN pull out..rare but possible...Bolts OTOH.. |
Coach bolts or coach screws?
On 29 May, 15:41, dg wrote:
Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - They're entirely different devices, for different purposes. One's a bolt, one's a screw. Bolts are plain-shanked dowels used to resist shearing (sideways) forces. For convenience in installation compared to a plain dowel, the ends are threaded so that a locking nut can be attached. Screws are compression fasteners intended to compress two pieces together. They shouldn't be loaded sideways in shear, as they're relatively thin and poorly attached, thus unable to resist this well. Obviously there's some crossover: bolts in particular can apply tensile forces, although screws are much less happy in shear. In principle though, use each for its intended purpose and don't confuse them. This applies equally to design in wood or metal, although wood is usually a little less fussy about applying shear to screws than metal would be. |
Coach bolts or coach screws?
On 29 May, 16:47, Andy Dingley wrote:
On 29 May, 15:41, dg wrote: Apart from reasons of practicality, is there any reason for not using coachscrewsinstead ofcoachboltsfor fixing large timber sections - They're entirely different devices, for different purposes. One's a bolt, one's a screw. Boltsare plain-shanked dowels used to resist shearing (sideways) forces. For convenience in installation compared to a plain dowel, the ends are threaded so that a locking nut can be attached. Screwsare compression fasteners intended to compress two pieces together. They shouldn't be loaded sideways in shear, as they're relatively thin and poorly attached, thus unable to resist this well. Obviously there's some crossover:boltsin particular can apply tensile forces, althoughscrewsare much less happy in shear. In principle though, use each for its intended purpose and don't confuse them. This applies equally to design in wood or metal, although wood is usually a little less fussy about applying shear toscrewsthan metal would be. I would have thought that say a 100mm long 10mm dia coach screw would have an almost equal shear strength to a similar sized coach bolt? I can see how a coach bolt with washers would resist being pulled out, and coach screw could potentially be weaker in this respect (limitations of the timber not the screw), but for lateral shear, both would seem equally adequate. dg |
Coach bolts or coach screws?
On 29 May, 15:41, dg wrote:
Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg For a start lets do the pedant thing and at least get them called by their right names - they are "coach bolts" and "carriage screws". The argument on sheer and tensile is cobblers as diameter for diameter the sheer strength is going to be the same. On the other hand the tensile strength of a bolted joint with a suitable load spreading plate is clearly always going to be greater than a screw into the wood. The choice is related to whether you can get at both sides of the joint - if you can then its a bolt - if you can't it's a screw. Can't really be simpler can it? Rob |
Coach bolts or coach screws?
On 29 May, 15:41, dg wrote:
Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg Basically screws are for fixing a thing TO a thing, bolts for fixing things TOGETHER. cheers Jacob |
Coach bolts or coach screws?
On 29 May, 17:10, dg wrote:
I would have thought that say a 100mm long 10mm dia coach screw would have an almost equal shear strength to a similar sized coach bolt? Perhaps, but a screw in a hole has much less _overall_ shear strength than a similar bolt. It's not about the screw being weaker, or even the core of the screw being smaller than the overall diameter, it's more about the relative strength against shear of screwthreads vs. plain shanked dowels. For wood it's less of a problem as wood isn't especially strong against any shear force applied to a narrow steel dowel. For metal it's significantly different -- no competent design shear-loads a screwthread when it ought to be using a plain-shanked bolt, often in a reamed and tightly fitting hole. |
Coach bolts or coach screws?
In an earlier contribution to this discussion,
Andy Dingley wrote: On 29 May, 17:10, dg wrote: I would have thought that say a 100mm long 10mm dia coach screw would have an almost equal shear strength to a similar sized coach bolt? Perhaps, but a screw in a hole has much less _overall_ shear strength than a similar bolt. It's not about the screw being weaker, or even the core of the screw being smaller than the overall diameter, it's more about the relative strength against shear of screwthreads vs. plain shanked dowels. For wood it's less of a problem as wood isn't especially strong against any shear force applied to a narrow steel dowel. For metal it's significantly different -- no competent design shear-loads a screwthread when it ought to be using a plain-shanked bolt, often in a reamed and tightly fitting hole. It's all a bit academic anyway, because if you hold two bits of wood tightly together - be it with a bolt or screw - any shear force if provided by the friction between the mating surfaces rather than by the bolt or screw. -- Cheers, Roger ______ Email address maintained for newsgroup use only, and not regularly monitored.. Messages sent to it may not be read for several weeks. PLEASE REPLY TO NEWSGROUP! |
Coach bolts or coach screws?
On 29 May, 17:59, robgraham wrote:
The argument on sheer and tensile is cobblers as diameter for diameter the sheer strength is going to be the same. I'll take Machinery's Handbook as authoritative here, over someone who confuses "shear" and "sheer". The choice is related to whether you can get at both sides of the joint - if you can then its a bolt - if you can't it's a screw. That's a question as to whether you're using a loose nut or not, not what type of fastener you've used. A bolt (albeit not a coach bolt) can use a fixed nutplate or a tapped hole and be inserted by rotating the bolt from one side. Conversely a parallel-threaded screw can have a nut applied to it. |
Coach bolts or coach screws?
On 29 May, 19:00, "Roger Mills" wrote:
It's all a bit academic anyway, because if you hold two bits of wood tightly together - be it with a bolt or screw - any shear force if provided by the friction between the mating surfaces rather than by the bolt or screw. Not good design though. It's very dificult (i.e. impossibly impractical) to maintain compressive forces and thus friction in a design of bolted timber. Moisture or thermal movement, then compressive yield in the timber, cause such a joint to go loose and fail after a few seasons. |
Coach bolts or coach screws?
In an earlier contribution to this discussion,
Andy Dingley wrote: On 29 May, 19:00, "Roger Mills" wrote: It's all a bit academic anyway, because if you hold two bits of wood tightly together - be it with a bolt or screw - any shear force if provided by the friction between the mating surfaces rather than by the bolt or screw. Not good design though. It's very dificult (i.e. impossibly impractical) to maintain compressive forces and thus friction in a design of bolted timber. Moisture or thermal movement, then compressive yield in the timber, cause such a joint to go loose and fail after a few seasons. True, but you can mitigate against that by using a 'spikey plate'[1] between the 2 bits of wood. That will keep the shear force away from the bolt even if the full compressive force isn't maintained. [1] Not sure of the technical term! -- Cheers, Roger ______ Email address maintained for newsgroup use only, and not regularly monitored.. Messages sent to it may not be read for several weeks. PLEASE REPLY TO NEWSGROUP! |
Coach bolts or coach screws?
Roger Mills wrote:
In an earlier contribution to this discussion, Andy Dingley wrote: On 29 May, 17:10, dg wrote: I would have thought that say a 100mm long 10mm dia coach screw would have an almost equal shear strength to a similar sized coach bolt? Perhaps, but a screw in a hole has much less _overall_ shear strength than a similar bolt. It's not about the screw being weaker, or even the core of the screw being smaller than the overall diameter, it's more about the relative strength against shear of screwthreads vs. plain shanked dowels. For wood it's less of a problem as wood isn't especially strong against any shear force applied to a narrow steel dowel. For metal it's significantly different -- no competent design shear-loads a screwthread when it ought to be using a plain-shanked bolt, often in a reamed and tightly fitting hole. It's all a bit academic anyway, because if you hold two bits of wood tightly together - be it with a bolt or screw - any shear force if provided by the friction between the mating surfaces rather than by the bolt or screw. Too true Roger - eveybody forgets that bit. -- Dave The Medway Handyman www.medwayhandyman.co.uk 01634 717930 07850 597257 |
Coach bolts or coach screws?
robgraham wrote:
On 29 May, 15:41, dg wrote: Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg For a start lets do the pedant thing and at least get them called by their right names - they are "coach bolts" and "carriage screws". The argument on sheer and tensile is cobblers as diameter for diameter the sheer strength is going to be the same. On the other hand the tensile strength of a bolted joint with a suitable load spreading plate is clearly always going to be greater than a screw into the wood. The choice is related to whether you can get at both sides of the joint - if you can then its a bolt - if you can't it's a screw. Can't really be simpler can it? And you can't easily use a bolt to make a tee joint. -- Dave The Medway Handyman www.medwayhandyman.co.uk 01634 717930 07850 597257 |
Coach bolts or coach screws?
The Natural Philosopher wrote:
dg wrote: Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg Personally I prefer bolts,, but screws are pretty damned good. Screws CAN pull out..rare but possible...Bolts OTOH.. I use loads of the Screwfix Turbo Coach Screws https://www.screwfix.com/app/sfd/cat...67177&id=46700 2 or 3 of those in 6 x 2 timber and it aint not going anywhere. Incredibly strong joint. -- Dave The Medway Handyman www.medwayhandyman.co.uk 01634 717930 07850 597257 |
Coach bolts or coach screws?
In an earlier contribution to this discussion,
The Medway Handyman wrote: The Natural Philosopher wrote: dg wrote: Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same? dg Personally I prefer bolts,, but screws are pretty damned good. Screws CAN pull out..rare but possible...Bolts OTOH.. I use loads of the Screwfix Turbo Coach Screws https://www.screwfix.com/app/sfd/cat...67177&id=46700 2 or 3 of those in 6 x 2 timber and it aint not going anywhere. Incredibly strong joint. Yes, they're good. Pity that even Screwfix doesn't know the difference between a *coach* bolt and a *carriage* screw! g -- Cheers, Roger ______ Email address maintained for newsgroup use only, and not regularly monitored.. Messages sent to it may not be read for several weeks. PLEASE REPLY TO NEWSGROUP! |
Coach bolts or coach screws?
On Tue, 29 May 2007 19:58:59 +0100, "Roger Mills"
wrote: True, but you can mitigate against that by using a 'spikey plate'[1] between the 2 bits of wood. You'll see me using "spikey plates" after they pry my twybill from my cold, dead fingers.... |
Coach bolts or coach screws?
On 29 May, 19:58, "Roger Mills" wrote:
In an earlier contribution to this discussion, Andy Dingley wrote: On 29 May, 19:00, "Roger Mills" wrote: It's all a bit academic anyway, because if you hold two bits of wood tightly together - be it with a bolt or screw - any shear force if provided by the friction between the mating surfaces rather than by the bolt or screw. Not good design though. It's very dificult (i.e. impossibly impractical) to maintain compressive forces and thus friction in a design of bolted timber. Moisture or thermal movement, then compressive yield in the timber, cause such a joint to go loose and fail after a few seasons. True, but you can mitigate against that by using a 'spikey plate'[1] between the 2 bits of wood. That will keep the shear force away from the bolt even if the full compressive force isn't maintained. [1] Not sure of the technical term! -- Cheers, Roger ______ Email address maintained for newsgroup use only, and not regularly monitored.. Messages sent to it may not be read for several weeks. PLEASE REPLY TO NEWSGROUP! Timber connectors dg |
Coach bolts or coach screws?
On 29 May, 17:59, robgraham wrote:
For a start lets do the pedant thing and at least get them called by their right names - they are "coach bolts" and "carriage screws". I thought they were just different words for the same thing - after all isn't a carriage a coach? Tomatoes, tomartoes? dg |
Coach bolts or coach screws?
On 29 May 2007 16:08:41 -0700, dg mused:
On 29 May, 17:59, robgraham wrote: For a start lets do the pedant thing and at least get them called by their right names - they are "coach bolts" and "carriage screws". I thought they were just different words for the same thing - after all isn't a carriage a coach? Tomatoes, tomartoes? You see, one of those is a made up word and one isn't, that's a different thing altogether. -- Regards, Stuart. |
Coach bolts or coach screws?
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Coach bolts or coach screws?
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Coach bolts or coach screws?
Roger Mills wrote:
[1] Not sure of the technical term! http://www.screwfix.com/app/sfd/cat/...99094&ts=05307 -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
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