by Graham Jost
This matter, or rather the problems associated with it, have been much discussed on Spanner recently. As a result of many contributions, a generalised account of the securing of bosses to axles using set screws or grub screws is given first, followed by the matter as it can be addressed in Meccano model-building. No reference is made here to fixings by key or pin - these totally positive methods are not applicable to the Meccano scene.
In tightening the screw in a boss down upon an axle, the axle is forced to the opposite side of the bore. Friction there, and that between screw head and axle act together to resist slippage between axle and boss. With increasing torque applied to the boss, slippage will eventually occur, as all Meccanomen know only too well! Increasing the tightening of the screw will provide a higher resistance to slippage (static friction is proportional to applied load) but there is a limit as to how far this can be increased before, eg, the screw thread is stripped, or the screw itself is broken.
But the situation can be improved by the use of a second screw in the boss. There are two choices here: the second screw can be in the same plane as the first, and at an angle to it, or it can be beside the first.
1) For in-plane screws, the resolved force from the two screws acts at an angle between them to increase the friction between boss and axle and, in addition, there is now the additional resisting couple between the second screw tip and axle. Theoretically, the angle between these two screws should, in order to maximise the resolved force (and hence resistance to slippage) be as small as possible - but will be limited by the requirement that they do not interfere at the bore - this included angle is often observed in practice to be about 90 degrees.
But there is another most important reason why this angle is chosen: it has to do with the matter of the securing screws becoming loose in service. Having tightened the first screw, and then the second - should either screw work loose, the other may still be expected to provide reasonable resistance to slippage - at least until the matter can be addressed. For screws at other angles, this benefit will be less marked: for angles more or less than 90 degrees, it will depend on whether the first or second screw tightened comes loose first - if the second, the situation reverts to the single screw fixing case, but if the first tightened screw loosens first, all grip, for practical purposes, is lost immediately.. The three-point fixing achieved by using two screws here is fully stable without any redundancy, and is an example of that often used in other engineering applications requiring "necessary and sufficient" conditions.
2) If the boss is sufficiently long, a second screw can be installed beside the first. In this case it is important that it be at the same angular position as the first, so that its effect - to virtually double the resistance to slippage - is maximised. In principle, there is no reason why a third, or fourth, screw cannot be added to achieve further improvement, but the boss by this time is beginning to become rather unwieldy!
Before proceeding further, a few words about Meccano Set Screws and Meccano Grub Screws are in order:
1) Meccano Set Screws come in two basic types - machine-threaded and roll-threaded. The former were manufactured in early Meccano days only, and have a flat or slightly convex tip, ie one which can indent the axle slightly when tightened. As a result of the rolling process, rolled Set Screws have a raised edge around the perimeter of the tip, which, in use, becomes flattened down relatively easily (Set Screws are certainly made of nothing more than what is known as mild steel). If this flattening occurs in use, then the screws will loosen quickly - it is better to ensure that this process has occurred before using such screws in situations requiring troublefree running.
2) Meccano Grub Screws come with a similar convex tip, or with a pointed tip, or with a hollow cone tip. When tightened down onto the axle, these screws can indent the axle either elastically only, in which case they leave no permanent deformation, or they can literally "bite" into the axle, to leave a permanent depression - the latter screws have such tips expressly for the purpose of gripping the axle in this way. In severe cases, the damage to the axle is sufficient to render difficult the subsequent sliding of a bossed part past the damaged region. The use of screws with pointed or coned tips is far more effective at securing the boss than is that relying on friction alone, and is more akin to the positive mechanical fixing discussed at the end of this note.
No Meccano bosses contain two tapped screw holes in the same plane at right angles. Here, because the outer diameter of the tapped hole and the axle are much the same, such tapped holes would certainly provide interference to the securing screws at the bore. But all Meccano bosses (post about 1928) are double-tapped, ie they contain tapped holes at 180 degrees. So what is the situation concerning the use of either one or two securing screws - should we use one or two screws to maximise the grip of the boss on the axle? There is no straightforward answer to this question deducible by reasoning alone. Simple tests indicate that, under static conditions, it is, indeed, better to use two screws - the use of two screws provided an increase in the torque required to produce slip of about 50%. But this static situation is not that which occurs in practice, and it is not that which gives rise to the problem in practice - here the matter is very much associated with repeated loadings, in which relatively tiny elastic deformations and movements occur with each rotation of a loaded wheel between axle and boss, and it is these tiny movements which can give rise, eventually, to one or other of the securing screws becoming loose. As to whether one or two screw fixing is best for our purposes, we must be guided by personal experience.
There are other points of interest concerning Meccano fixings.
1) One screw only: this leads to offset, non-concentric axle and boss. Clearly, the closer the fit, ie the smaller the clearance , between bore and axle, the better, and the less will this be of consequence. Excess clearance could lead to difficulty in gear or chainwheel drives, for example - in extreme cases, cyclic binding of gears could result, or excessive cyclic tightness and looseness in chain drives could occur. On the other hand, by careful choice of fixing hole and axle, this fact could be used to minimise the eccentricity in a mounted, but already-eccentric, wheel.
2) Two screw fixing: here we have two choices, either by even tightening of the screws, or by tightening one fully first, and then the second. Even tightening of the screws will give rise to the axle being held between them, in principle clear of the bore - the axle could actually be concentric with the boss. But it is supported only between the tips of the screws - there is no three point fixing, and it will be relatively unstable to applied fluctuating loads. Tiny relative movement between axle and boss is easy to imagine on a rotation-by-rotation basis, leading to relatively early loosening of the screws and loss of grip. The alternative tightening sequence should be far better from the point of view of limiting relative axle/boss movement, resulting in longer trouble-free running.
The above aims to summarise the situation in Meccano, so long as no mutilation is permitted. But there is a most effective, and well-proven technique available when all else fails: this is to file a modest "flat" on the axle, against which the securing screw is then tightened. In this case, we have a mechanical fixing of a type commonly used in light engineering, which does not rely at all on friction for its effect. This fixing technique will be found to provide long, if not indefinite, periods between required retightening. In this respect, the new Meccano triple-flat axles should solve completely the problem of the secure attachment of bosses to axles - the requisite "flats" are already built-in!
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