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Substitutes for parts 142 and 155

(Author: Paulo Kroeff de Souza)

Most rubber parts and the nº40 cord are among the few “expendable” Meccano parts. Tyres, mounted permanently on an extra set of wheels, will be out of this set, since staying on wheels, they will last for decades. I have adopted that solution. So, we are concerned with part 40 – cord, parts 142 and 155 – rubber rings and parts 186X – driving bands. This text is about the rubber rings.

Looking for an alternative supply for rubber rings 142, for 3” pulley 19b, and 155, for 1” pulleys, I tried “Mercado Livre” which is a Brazilian site similar to E-Bay. I found a seller, “Retentive” (www.retentive.com.br), which supplies “O”-rings of various rubber compounds in some special dimensions. After some dialogue they offered me existing sizes in Nitrile rubber (hardness Shore 70) characterized by internal diameter (ID) and “thickness” (T) from which the outside diameter (OD) can be calculated.

I started with the dimensions given for the original Meccano parts in Meccano engineering drawings available from Tim Edwards site, listed by part number: https://www.meccanoindex.co.uk/Drawings/Parts.php?id=1580657130.
part 142: ID=2½”=63.5mm OD=31/8”=79.375mm T=5/16”=7.94mm
part 155: ID=27/32”=21.43mm OD=13/16”=30.16mm T=11/64”=4.37mm

Among the sizes available from the supplier, I found the following interesting:
ID=68mm OD=82mm T=7mm (for simplicity let’s call it the “68” ring)
ID=70mm OD=86mm T=8mm (for simplicity let’s call it the “70” ring)
ID=22mm OD=30mm T=4.5mm (for simplicity let’s call it the “22” ring)

To evaluate the match of the part the best way is to calculate the volume of rubber in the ring because, when it is stretched over the wheel, it will very approximately maintain its volume. A ring of circular section is a torus and the formula for its volume is V= πD(πd2/4), where d is the diameter of the section (called “thickness”, T above), and D is the mean diameter of the ring D=(ID+OD)/2. But it is far easier to go to the page: https://keisan.casio.com/exec/system/1223388402 of the Keisan on-line calculator site where entering internal radius and external radius, you have the volume very precisely calculated, with no effort besides dividing the diameters by 2...
I obtained approximate results by entering radii with 4 decimals and rounding the volume to the nearest mm3. For the dimensions of the Meccano parts we obtain:

part 142 V=11105mm3 part 155 V=1213mm3

The “68” ring above has a volume of 9067mm3 or 18% less than part 142. This means that when stretched over the wheel the “thickness” will be some 10% smaller or 0.7mm. The “70” ring has a volume of 12317mm3 or 11% more than part 142. So, when stretched its “thickness” will be some 4% “fatter” than the original Meccano part. Since I prefer it to be thinner, I bought the “68” ring. The final “practical diameter obtained over a 19b pulley is 87.5mm, the ring “thickness being 6.9mm.

The “22” ring has a volume of 1324mm3 or 9% more than the original 155 ring. This means that when stretched over a pulley it will be only some 3% fatter than the Meccano part, some 14 hundredths of a millimetre, which will be unnoticeable visually. The final “practical diameter obtained over a 22/22a pulley is 32.9mm, the ring “thickness being 4.4mm.

Then I decided to equip with rubber rings also the 1.5” pulleys 21 and the 2” pulleys 20a. To adequately size those rings, we have to consider the width of the groove of the pulleys and compare them with the 19b and 22/22a parts. Back to the Meccano drawings at Tim Edwards site we find that pulleys 20a and 21 have the same specified groove width as pulleys 22/22a or 3.96mm. However, measuring the groove width of some of my pulleys I found quite a lot of variation, some of them having wider grooves up to 5mm for some 20a’s. Sticking to the drawings, the ring nominal “thickness” will be, approximately, 4.5mm. In practice it might be better to have fatter rings chiefly for the 20a pulleys.

To arrive at the ID of the rings, we may use the same proportions as for 22/22a pulleys. Those have an OD of 1” or 25.4mm and the nominal 155 ring ID is 27/32 or 21.4mm. So, the ring OD is 0.84 times the wheel OD. Based on that, the ideal ID for the rings for the 21 pulleys would be 32.1mm and for the 20a pulleys, 42.8mm. Back to the supplier I got an offer of ID=33mm, T=4mm for the 21 pulley, which is somewhat thin, and ID=43.82mm, T=5.33mm which looks ideal for the 20a wheel. The final diameter of the ringed wheels are 44mm for the 1.5”, 68.6mm for the 2” wheels. The final width for the rings are 3.8mm for the 1.5”, 5.1mm for the 2” wheel.

Another interesting exercise is to fit the “68” ring to the spoked wheel 19a which is a delicate but possible operation, since holding the wheel by the spokes may warp it.

The final results may be appreciated in the photos below.


 

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