On the 4-speed milling head, the tufnol gear on the layshaft assembly has damaged teeth. Despite inspection, I can't see how the tufnol gear is mated onto the assembly - is it simply a force fit, removable with a bearing puller?
I'm attempting a repair but am nervous about damaging the needle bearings within the assembly, so would like to be reasonably sure about being "right first time"
Well, I looked at the manual...........It doesn't show how the cluster gear is assembled. Sorry. I do not believe that it is a press fit alone since the fibre gear material would relax over time; further, it would not carry the high hoop stress necessary to transmit the requisite torque.
Clean and examine the assembly carefully and see if you can find any shear pins, rivets, press-fit splines (very fine splines, almost axial knurls), etc.
If all else fails just obtain a replacement fibre gear, then machine the assembly to remove existing. Machine new gear to fit and fasten using pins, etc.
The Tufnol wheel is held on by micro-splines and what appears to be Araldite (Britland epoxy resin). I made up a mandrel so that I could employ a bearing puller and the old wheel just slipped off (I also made up an FG washer for the puller to purchase on so that there was no force on the individual teeth)
Unfortunately, estimating how much smaller a hole has to be to be an interference fit over the splines was not my lucky day!
My new gear wheel split when it was half way over the splines, so time to think again.
Also, the pressure applied seems to have distorted one of the sets of needle bearings as it now feels rough. No doubt when originally manufactured, the needles were slipped in last, but in this case, the internal circlip doesn't have any holes to get a purchase on with a pair of circlip pliers, so they had to stay in.
ISTR reading somewhere that some of the smaller mills use nylon gears in the power train.
Is this so?
How does nylon react to the oils and greases in the gearbox?
Is nylon self-lubricating as is PTFE?
The reason I ask is that I've got a cylindrical lump of nylon in the junk box, and it has occurred to me that I could try to refabricate the whole unit, rather than just one of the gears, out of nylon.
I don't know what tufnol is, but the reinforced material you want to use for a gear is fabric reinforced SHEET material, ie. many layers of cloth building up the thickness, impregnated with phenolic, then cured.
Do NOT use rod-type material (where the cloth is spirally wound) as this would produce weak gear teeth.
The fine press-fit splines on the spindle are a pain....Suggest you turn some test pieces of gear material with differently sized bores and press them onto the spindle. when you get to the one that doesn't break.....you have your answer.
Sounds very much like shades of a fettled casting.
However, as is usual for this poster, small but fundamental errors creep in at an early stage. In this case it occurs in the (mistaken) throwaway remark that Araldite (is) "Britland epoxy resin"
In order to disambiguate the chappy, I append a small FAQ on the discovery and early commercial history of Araldite resins.
There is plent of information on the web concerning this fascinating topic, but as this chappy 'doesn't follow web links' there is little point in putting them up.
"Epoxy resins were first synthesized by Blumer and patented in Germany in 1930, Another patent by Schlack was issued in 1933 also in Germany. DeTrey Freres in Switzerland are credited with the first production of Epoxy resins which they licensed to Ciba AG in the early 1940's. Ciba AG (now Ciba Geigy) demonstrated this product under the Trade Name Araldite at the Swiss Industries Fair in 1945. The use of the Araldite name suggests that the original use was as an adhesive but the original search was for new paint resins and that is by far the largest use of the epoxy resins now."
The above was extracted from a paper by Raymond B Seymore, published by Elsevier in 1990.
Interestingly, Novartis was created in 1996 from the merger of the Swiss based life sciences companies, Ciba-Geigy and Sandoz. The name 'Novartis' is derived from the Latin novae artes - new skills. Perhaps this would be a suitable route for the OP to try to follow. He could start by researching the recent commercial history of Araldite, for example.
I append a small FAQ on some relevant considerations for this application, in which you will note that any self-lubricating properties of nylon amount to little in the light of the more relevant material properties. In other words, you may have asked the wrong question.
" General Information for 'Nylons'.
A family of similar semi-crystalline engineering thermoplastics generally synthesised from straight chain aliphatic acids and amines. Each particular nylon is described by one or two numbers eg nylon 6 or
6,6 where the number is the number of carbon atoms in the amine and acid monomers. Where a single number is used the monomer is an Alpha amino-acid (or its lactam) and the amide links in the polymer are all "head-to-tail". Where two numbers are used the monomers are a diamine and diacid and the amide links alternate "head-to-tail"/"tail-to-head". The first number refers to the diamine, the second to the diacid. Some nylons use aromatic monomers which are denoted by letters eg T for terephthalic and I for isophthalic components.
Nylons generally are strong, tough (but notch-sensitive), rather resilient polymers with good barrier properties and high fatigue and abrasion resistance. However, they have high water absorption (slow in thick sections) accompanied by an increase in dimensions which can be as much as 3% under extreme circumstances. Nylons have good resistance to oils, greases, solvents and alkalis but not to acids which tend to hydrolyse them. The most common nylons are 6 and 6,6 and in the individual descriptions the others are described in relation to them.
General Description of Nylon 6,6 : A semi-crystalline, off-white engineering thermoplastic that is the strongest and most abrasion resistant unreinforced aliphatic nylon with better low temperature toughness than Nylon 6 or acetal. Its very low melt viscosity can give industrial processing difficulties and weathering can cause embrittlement and colour change unless it is stabilised or protected. Available with a wide range of fillers notably glass fibre, which gives a marked increase in stiffness, and solid and liquid (oil) lubricants. Super-tough grades are also available whose impact properties and low notch sensitivity are amongst the best of all engineering thermoplastics.
Applications include mainly engineering components eg gears, bearings, nuts, bolts, rivets and wheels and power tool casings and rocker box covers. Widely used as monofilament for brushes etc and fibre - notable for its resilience and high abrasion resistance - for apparel, carpet and industrial end-uses.
From this you will easily see that a random lump of 'nylon' in the junk box can be of any type or grade, but you will realise that only some of these, the higher-performing engineering grades usually with specialty fillers, will be the most suitable for the technical use to which you refer. Unless you know the provenance of your lump, you may be at high risk of severe diappointment if you use it in this way. This does not preclude you performing this exercise as a form of learning new skills.
Should you have broken the habit of a lifetime and followed the above link, your eagle eye will also have noted that the Oils and Grease resistance is noted as 'Good', which is not the same as 'Excellent'.
I leave it as an exercise for you to determine the relevance to you of the implications of the warning that "...Its very low melt viscosity can give industrial processing difficulties".
Thanks, that was very interesting. I was coming from the pov of RC model servos where I used to shun plastic gears after suffering a few breakages but then I found the metal gears eventually wear and then backlash is a problem. Larger plastic geared servos aren't as fragile as the smaller ones and have lower backlash since the gears can be set tighter than the metal ones owing to the higher compliance you mentioned. I had always assumed they were nylon gears but they may be acetal for all I know.
I have _NEVER_ originated impolite personal remarks.
I have on occasion, asserted the right of reply, and in such cases (of which this is perhaps a case in point?) I find myself in a Harrisian Transaction playing the adult engaged in chastising someone else who is playing the child.
No problem, glad to expand upon my thoughts on the matter. Can I start by saying I am not anti-plastic, in fact quite the reverse is true. My life has been in the plastics industry and we are constantly working with components we are required to convert from metal to plastic. However, on many occasions we will look at a project and advise to have it cast, stamped or machined, as polymers may not be entirely suitable for the particular application. Geared head drive on a milling machine would be one IMHO.
A previous poster has already quoted some of the reasons. Dimensional stability due to atmospheric change is one. Nylons are hygroscopic and can absorb up to 8%-9% moisture in saturated conditions, and typically
2%-3% in a normal atmosphere. The process is reversible and it will give this up in a dry atmosphere, but the material will swell and shrink during the course of such a cycle. Unless you can use a speciality hydrolysis stable nylon then the dimensional change under normal equilibrium conditions would be around 1%, so a gear with a pitch diameter of say 50mm may grow or shrink by 0.5mm. By contrast, aromatic hydrocarbons & lubricating oils, greases etc. have little or no effect.
Water absorption has a secondary effect too, in that it will increase the toughness and ductility of the material, but at the expense of surface hardness and stiffness. Which brings me neatly on to the next point.
The stiffness, or Youngs modulus, of the material is the next point to consider. On average, the Youngs modulus of steel is probably 100 times greater than (dry) Nylon, at around 200Gpa to 2Gpa. The material is inherently compliant at this level unless you go to reinforcing fillers such as chopped glass strands (100-300 microns in length) added at anywhere from 15%-40% by weight to the polymer. However, increasing the stiffness also increases the brittleness of the material, and additionally glass fibre reinforced materials are more notch sensitive because of the fibres.
So far we have a material that can swell and shrink and become softer or harder dependent on the weather. Albeit we are talking small amounts, and the figures all come from pretty standard test bar sized specimens, but now we can look at more specific geometry, i.e. the gear tooth.
At 1.5 module, (as he mentioned) the tooth size will a bit over 2mm wide at the pitch circle decreasing with the pressure angle so its thinner at the tip. Fine if you can ensure that the gears always mesh as they should with the point of pressure tangential, but if you get a size change due to moisture you may get a situation where the point of pressure moves closer to the tip and the tooth form becomes more of a cantilever. In this situation the Nylon tooth form can either smear or break depending on its wet/dry condition and the force or shock loading applied to it. This can even happen with the correct mesh too. There are calculations for the resultant force on a gear tooth that can prove (or disprove) this, but I don't happen to have them to hand at the moment!
And these are just the main points. I haven't even considered the effects of temperature, or creep, or relaxation in the material.
The main reason for producing gears in polymer materials is that both plain and incredibly complex forms can be mass produced very cheaply by injection moulding, and the inherent compliance in the materials mean they will generally run much quieter than ferrous or other metallic gears. ideal for instrument and office type applications. Nylon itself is a relatively poor choice for anti-friction properties unless it is compounded with low friction additives. The Nylatron mentioned by Martin is loaded with Molybdenum Disulphide, and compounds formulated with Silicon Oils and PTFE are also available.
Acetal resins (POM) have the lowest CoF of all polymers apart from PTFE, and pound for pound are the stiffest unfilled materials you can get. They are largely unaffected by moisture, and being very highly crystalline generate a superbly smooth, hard (relative!) surface. If I were ever to make any plastic gears I would choose this.
Peter (sorry - I said it was long..)
Hmmm??so this wasn't you talk>I hate it when those who cannot count suggest that a quadrasyllabic
Not Harrisian, OM, Bernian. Harris merely produced the "pop psych" version dumbed down for the great unwashed. I think you will find that there are others, such as Ian Stewart and Van Joines, that did a far more thorough and coherent job of presenting the modern interpretation of Berne's work.
And I believe the term you are looking for to describe your favoured transactional role is "Critical/Controlling Parent" not "Adult".