Member postings for John Fielding

Hi Clive,

Unfortunately your formula is wrong.

The standard method is to subtract the thread pitch from the diameter to obtain the maximum thread depth. Hence M3 x 0.5 is 2.5mm and a M4 x 0.7 is 3.3. But as Prof Chaddock pointed out years ago this gives 95% thread depth which is not warranted in many cases. For soft materials such as aluminium about 80% depth is adequate for 90% of the potential tensile strength. For harder material such as mild steel then as little as 60% thread depth is more than adequate. So a drill of 3.3mm to 3.6mm is the range for a M4 x 0.7mm thread.

Hi Norm,

Point taken. Perhaps my poor choice of words, maybe change "become" to "remain" would have been a better choice.

Interestingly I dug out a book on the Jaguar V12 engine development and it showed that flow rates at +90C and +120C gave a higher flow rate at the higher temperature using 20W-50 oil. I suppose that makes sense as the hotter oil would be a bit thinner and easier to pump?

Hi Ron,

I really sympathise with the problems you are having. I only picked up on this thread now but it seems as if it has been going on for a while.

You are right about reheating a boiler to cure leaks, not recommended as you fix one small leak and create two more! So if it were me I am afraid I would have demanded my money back a long time ago and then asked for advice from club members who they would recommend.

Boilers are potential bombs if incorrectly constructed and the workmanship isn't up to scratch. I have a friend who is a coded welder with the highest ticket for pressure vessels and some of the tales he has told me are scary!

Something puzzles me about this whole saga.

OK I haven't followed it from the beginning so I must be missing something, but how in blazes could it have been tested without the blanking plugs etc in place? Surely the constructor had to bung up all the holes before it was pressure tested. So why must the purchaser have to make and fit his own for a repeat test? Doesn't the constructor have the original plugs etc? Which begs the question, how certifiable is the certificate supplied?

My two cents worth - I get the impression that this company is not the best one to make and supply pressure vessels, I wonder if other customers have had a similar experience?

Does the item not come with a users manual? I would have thought that would be a good place to start.

It also highlights an apparent problem with the traditional oil boxes on loco connecting rods which are on the top. Oil holes in the top of axle boxes as we make them are also not the correct place to put them. However, on the scale of model locos compared to full size ones the shaft diameter is much smaller, so the peripheral speed, which is the determining factor, has to be much higher to have a severe effect. As LBSC said "You can't scale nature".

OOPs - sorry typo. Mind wandering as the ears were on the GP!

Perhaps this is common knowledge to some, but I often wondered what do all those numbers and letters mean when an oil is defined? We sort of know that a SAE 40 oil is thicker than SAE 20 etc but what does the number really define.

SAE is short for Society of Automotive Engineers, an American body who define standards which are recognised throughout the world. The number refers to time in seconds!

The viscosity of an oil is determined by pouring a known quantity through a calibrated aperture and the time it takes is an indication of the viscosity, thicker oils take longer to pour than thinner oils. A standard instrument that contains a calibrated volume of oil is used with a calibrated aperture.

SAE standards cover 20, 30, 40, 50 and 60 and the temperature the oil is tested at is +100C. The thinner grade oil such as 10, 20 which have a W suffix is tested at -10C. So a 10W - 50 oil has a pour point viscosity of 10 at -10C and a viscosity of 40 at +100C, that is it is modified to become thicker as the temperature rises.

The things you learn from books!

Hi Michael,

Yes it obviously is well known, if that is your field, but it is the first time I have come across it and I thought other readers might be interested!

I found it in a mech eng textbook by Prof Robert C Juvinall of the University of Michigan, one of the ones I bought at a close-out sale of a secondhand bookshop recently.

The other thing I learned was how viscosity is defined and what all those numbers, like 20W-50 means. Very interesting topic!

Hi Frank Brown,

There is a simpler method!

The voltage drop between no load and some current if you can accurately measure it is a good indication of the ESR. You just need to be able to measure the two voltages and the current accurately and then apply Ohms Law.

Hi Ajohnw,

You are correct about terminal voltage and car alternators. GM some years ago were persuaded to change from standard SLI types to lead-calcium and found they had a flood of complaints from owners because they didn't hold the charge. So they had to redesign the IC regulator in the alternator to raise the terminal voltage from about 14.0V to 14.4V to suit the new battery technology. Now the danger is that if the battery supply shop - who are almost clueless about things technical in many cases - sell you an old style battery, it will boil with the higher cut-off voltage.

Same thing if you have an older vehicle with a 14.0V alternator, and a new technology battery is fitted. It will never get the battery into a fully charged state. Most owners who use their cars every day will probably not detect a problem, but go away for a couple of weeks and then try to start your car and it will be pretty flat.

Hi Here Again,

Sometime ago I wrote an article for ME which showed a simple method of protecting the leadscrew on my Super 7 lead screw which is low cost.

In automotive harnesses they use a flexible plastic conduit which is split along the length and is available in several sizes. If you cut a piece of this and open it up along the slit you can wrap it around the lead screw. Being corrugated it can telescope a fair bit when it reaches a stop. I changed the Mazak thing Myford fitted to protect the lead screw and made a strip of ali which replaced it and added a hollow bush attached at the far end to run over the crest of the thread so it slid freely and that keeps the whole thing aligned. The spaghetti tubing is fastened by a nylon cable tie to the ali strip near the carriage and near the far end . When the spaghetti gets too messy I simply put a new length one.

A while ago I mentioned the odd effect of the headstock oiler. When running the oil level in the cup rises, but when the spindle is stopped it drops significantly. Several readers confirmed their lathe did the same. I found the answer in a textbook recently and the reason is very interesting and has an important message about oilers and where on a bearing housing they should be.

In the 1860s hot axle boxes on locos and rolling stock was a common problem and several changes were made to alleviate the problem. One was to cut an oil well into the bottom of the axle box bearing to hold oil. But this didn't solve the problem. A British railway engineer Beauchamp Tower decided to investigate the problem.

In a railway axle box the axle shaft only contacts the top surface of the bearing, it never contacts the bottom of the bearing. So the oil well should be the ideal place to place it. Tower discovered that oil was squeezed out from the bearing when spun at high speed. To resolve the problem he made a simple test apparatus which is shown here.

Tower drilled an oiler hole in the top of the bearing intending to try different oilers. However, he stuffed a cork into the hole as he wasn't at that stage in the experiments. When he spun up the axle the cork popped out! He hammered it in and it did the same thing. He then used a piece of wood and hammered it in good and hard. It was blown out like a champagne cork when the axle was spun up to speed. He then attached a pressure gauge and was amazed at the pressure indicated.

The reason is that the axle shaft in a plain bearing sleeve always runs eccentrically as they has to be a small clearance between the shaft and the bearing. The shaft when stationary will contact the bearing and any oil present will be squeezed out due to the weight on the bearing. When the shaft begins to rotate any oil will be dragged around until the shaft reaches a certain minimum speed and then it forms a wedge of oil which is passing from a large gap to the small gap between the shaft and stationary bearing. The shaft then "floats" on the oil with no metal to metal contact.

It works in the same way as a water skier waiting to be towed. When the boat begins moving the skier leans back holding the rope and the skies are pointed upwards at an angle and as the speed increases the skies ride up through the water until they are sliding across the top of the water. This supports the skier by generating a high pressure region due to the wedging action of the liquid.

So it should be obvious that an oiler must not be placed at a high pressure point in a bearing unless the oil is fed in under pressure to overcome the pressure generated by the shaft rotating. Liquid will always flow from a high to low pressure region.

In America in the early railroad days it was common to need two or more locos to get a long and heavy train moving. But when the speed was above a certain limit the oil begins to float the axle shaft and the friction drops considerably. Below a certain speed the bearing is in metal to metal contact and has high friction, but once the speed rises the oil film begins wedging and building up pressure and the axle rise off the bearing and floats on the oil. This known as hydrodynamic fluid motion.

So the oil in a plain axle box is often held in felt pads in the bottom of the box so that a small but constant supply of lubricant is wiped onto the shaft but preventing the pumping action which empties the oil box. The oil is poured into the box with the shaft stationary by a filler cum level plug below the axle shaft.

With the invention of rolling bearings (ball and roller types) the friction in the axle bearings was reduced dramatically and starting long trains was not as big a problem.

So the Myford headstock oiler with the wick is doing exactly which is required as it is in a low pressure region of the bearing and the spindle builds pressure pushing the oil back to the oil cup. But the wick provides adequate oil with the wiping action to supply the bearing.

Edited By John Fielding on 19/03/2016 07:24:24

So "Model" Engineering would appear to be a hobby practiced almost exclusively by a very (very) small proportion of the middle-aged to old (predominately retired) male population of the UK

Its not just model engineering which exhibits this trend I am afraid. One of my hobbies is amateur radio which I have been involved in for as long as model engineering and the same problems arise there. How to attract young blood into the hobby, and I am sure many other hobbies suffer from the same problem. Without new blood coming into the hobby it is inevitably doomed, when all the old fogies pop their clogs there will be nobody left to keep the hobby going.

I think the mind set of younger people has changed over the years, quite radically in some cases and not always for the good. When most of us were growing up we had no internet, computers, video players etc but kids today accept it is the norm. When a grandson is taken to a steam rally and sees the machines they are confused and wonder how backwards we must have been.

I heard a lovely story about an old geezer in America attending his grandsons college football game as a spectator and sat next to him was a loud mouthed first year student. It went something like this:

Young student "You know your generation are so stuck in the dark ages it frightens me grandpa. I mean look at the things we have today you didn't have" And went one to list a whole list of things like cellfones, internet, jet aircraft etc. And then launched again into deriding the old people until he paused to take a swig of his drink.

At that the old geezer interjected" Yes young man things have changed and we now have all this wonderful modern contraptions, but guess who invented all the things you take for granted. It was my generation who invented all those things - you silly fool. Now what the f*ck have your generation done that comes close!"

Hi Muzzer,

Yep you are quite right!

The thing that gives me the giggles the most is the "Gas Gauge" trend. Almost all portable appliance which uses rechargeable batteries today seems to have one sort or another. But none of them work accurately and are easily fooled. I have tried a few of these ICs and although they do give some sort of guide they are at best a gimmick to sell the expensive toy.

To be able to predict with any degree of certainty how much capacity a cell or battery actually has takes a bit more than measuring the joules put in by charging and the assumptions made thereof. You would need to know the precise capacity of the cell during discharge and the environment and even then you could be out by 20% or more. You would also need to know the capacity degradation per charge-discharge cycle for that particular cell or battery, which you don't have as it is a new cell. Using data from another cell of the same type is no guarantee either, cells differ even from the same batch over quite a broad spread.

The basic problem with batteries is the user, most people know absolutely zip about them and salesmen take advantage of this fact and then its your money they are after and nothing more!

Hi Norm,

Yes, you got it spot on! It fools many people when the charger doesn't seem to work correctly and assume it puts out a smooth or pure dc voltage.

The reason the DVM gives an apparent wrong reading - it actually measures correctly with the waveform it is presented with - is because a half-wave sinusoidal pulse is not a sine wave. The same thing occurs with PWM square wave forms of varying duty cycle. If you use a moving coil meter and measure the voltage which is, say going from zero to 10V for example and the duty cycle is 20% on time and 80% off time, then it will read 2V. It averages the voltage because of the inertia in the movement. If you increase the duty cycle to 50% it will read 5V etc.

A DVM can give a totally different reading as it sample the waveform periodically and if the sampling rate is not the same as the applied waveform frequency then it gets confused. Suppose the sampling rate is exactly the same as the applied frequency, what will it measure? Well it could read 0V or it could read 10V, it depends where it catches the pulse!

I came across this when working on radar systems and a technician connected a RF power meter in line and was puzzled why it read so little power. I had to explain to him although it was a 10kW transmitter it was only transmitting for one micro-second every milli second, so the meter read virtually nothing, just a few watts indicated. When you calculated the duty cycle it was only 0.1% so the meter was correct.

So I am glad you have remembered your lessons from ancient history!

The other question I am often asked is "My battery I know is flat, but when I connect my charger it draws very little current. Surely if it is flat then it should draw maximum charge current?"

The answer is NO. And you need to understand how a cell or battery works. A cell can be modeled as a constant voltage source which can supply infinite current with no drop in voltage. The ideal cell in other words. But no cell works like that, as you load up the cell discharge current the voltage begins to drop. To explain this effect we need to include a "loss resistor" which is between the perfect voltage source and the terminals. As the discharge current rises, more and more voltage is dropped across the resistor, which we call the ESR (Equivalent Series Resistance". This cell model covers all the operating parameters of any cell type. So when the cell is fully discharged the ESR has risen to a high value and it can no longer allow sufficient current to flow to the load, most of the voltage is being dropped across the ESR.

Now when you now connect the cell onto a charger, the ESR is still a high value but after a little charge current has flowed into the cell it begins to drop as the cell recovers its chemical balance and so the charge current climbs to some maximum. This is known as the "Bulk Charge" region. When the cell is almost fully charged its voltage rises and equals the charger output voltage and the charge current falls towards zero. When a cell or battery is fully charged the ESR has reached its minimum value and the cell can deliver maximum current to a load.

Plenty of cells and batteries have been scrapped because of this effect. If they had waited an hour or so they would have seen the charge current begin to rise!

Hi Peter Shaw,

I began my career as a battery technician in the BPO back in the late 1960s. In those days a battery had to last a minimum of 8-hours in case of mains failure and the minimum life of a battery had to be not less than 15 years. If it croaked in say 12 years there was a huge spanish inquisition. A great deal of my time was doing recovery work on the exchange batteries the LTOs had neglected, because they would rather do other things than mess with stinky batteries!

Remedial work was a series of charge-discharge cycles to bring the battery back to near full capacity. Some were so badly neglected it took 2 or more weeks of work to get them back up into reasonable service.  A telephone exchange battery is nominally 50V dc and often positive ground.  It consisted of 24 cells connected in series with link straps.  The battery was normally doubled up so there were two identical rows of cells, one was in use and the other trickle charged as a back up.  Once a week the battery switch was swapped over so the other battery was the main battery and the one just out of service became the back up.  All the batteries had at least two spare cells awaiting fitting in case a cell went AWOL and it was common to find we had to swap out a dud cells with one of the spare ones whilst in standby or back up mode.  One old exchange I was involved in decommissioning was built in the early 1940s and it was a very small manual exchange and the battery was still perfect after all those years, because the maintenance was done correctly by a traveling technician who called once a week.

In bigger exchanges it was normal practice to also install standby generators, but the smaller exchanges didn't warrant the expense. The biggest battery system I worked on was the one in the Edinburgh trans-atlantic job. IIRC it is at Murrayfield or it could have been Muirhouse, it was a long time ago!

Forgot to add a bit about telephone exchange battery systems.

All the types in common use in my days at BPO were "float charge" systems. They may have had different types later but I was gone by then.  In a float charge system the main charger supplies the current requirement of the exchange and also kept the battery topped up.  In effect the charger and battery were simply connected in parallel, so if the mains failed the battery stepped in to take over the load, with no break in service.  The 8-hour parameter was a reasonable time period, in the UK the mains supply was pretty reliable and if an exchange was off-line for more than a few hours then we also had a number of vehicle mounted large diesel generators which could be sent to an exchange to provide backup if the battery was in stress with the load.  Not sure what the policy was prior to that but 24-hours does seem rather a long time and I never heard it mentioned?

Car alternator system work in exactly the same way, the alternator provides the current required and the battery steps in to offer assistance if the alternator output falls too low.  If it wasn't for the starter motor the battery could be a lot smaller.

Edited By John Fielding on 18/03/2016 09:13:41

Edited By John Fielding on 18/03/2016 09:24:40

Edited By John Fielding on 18/03/2016 09:33:55

Hi Frank,

Yes he was absolutely correct and I have seen that problem.

No matter what fancy modern chargers appear on the market the old transformer - rectifier system running off 50 or 60 Hertz still takes a lot of beating. The pulsing current does seem to be a better way of charging than a bench regulated supply which is pure dc. Many people have tried to find the reason but it eludes them. One theory, which I think seems the most logical explanation, is that the charging and discharging is a very SLOW process and the chemicals get sort of lazy and sluggish. But if they get hit by a rapidly changing current they seem to wake up. Westinghouse in the USA patented such a system and it does seem to work better. But changing out critical batteries every year is a wise plan, as long as the cost isn't too high.

And don't lose the little bitty ball on the other end! Ask me how I know

Iain,

Try using proper slitting discs in your angle grinder. I was introduced to them a few years back and have been using them ever since to chop stock bar material into lengths for the lathe. The thinnest disc is only 1mm so the wastage is about the same as a kaksaw (yep I did write that) and they walk through most materials but normal eye goggles etc recommended.