CHAPTER 2
LINKAGE MECHANISMS
90° Clevis
As an initial example of fabrication coupled with the use of a simple. jig
a 90° clevis is shown. This clevis, normally used for coupling ploughs
and farm implements to a tractor, enables an implement hitch and a tractor
hitch at right angles to each other to be easily coupled.
Fig
7
Fig
8
The original in Fig. 7 is a malleable casting, repaired by bronze welding. A stronger replacement has been made from mild steel bar. The method employed was to mark out the four lengths or material on a bar. The centres of the holes were then centre-punched to a greater depth than normal for drilling to form a recessed pivot point for the use ora trammel bar with the cutting torch.
Fig. 8 shows a trammel bar in use. The pivot point on the trammel bar is set so that the distance front the pivot point to the near edge of the oxygen cutting jet in the cutter nozzle is the radius of the rounded end needed on the clevis jaw. This precise setting of the trammel bar varies according to whether an outside curve or an inside curve is needed.
In the present case the cutting kerf is excluded and therefore the measurement is to the inside edge of the oxygen cutting jet. If the curve being cut is to form an inside edge, as in the case or piercing a hole or cutting a large washer, then the setting of the radius is from the pivot point to the outside edge of the oxygen cutting jet.
To avoid unnecessary cutting the four rounded ends can he cut as in Fig. 9 and the two lengths thus formed can then each be cut in half. The four parts are then set tip with the aid of two 'G' clamps and a small block of wood and strong tack welds made, see Fig. 10, left.
The 'G' clamps and the wood block can then be removed and welding completed with the welds in the gravity fillet position to ensure equal lengths of leg. This makes it easier to get a weld with a good profile and avoids reducing the section of the metal by the edges melting into the weld.
As the clevis is quite short in length this permits similar fillet welds to he made inside the box section as well as on the outside corners. thus greatly strengthening the whole fabrication. The clevis is then clamped in a vice in the vertical position and the two fillet welds at each corner, inside and out, sealed together with a short run.
Care must he taken to fill adequately the weld crater, as it is at this point that the maximum strain will be developed. I he welding completed, the clevis pin holes are drilled. This mild steel clevis is stronger than the original malleable casting: it can be rebuilt when the holes are worn oval in use and straightened in the fire when accidentally bent. In short it should now be a permanency and not a part needing replacement from time to time.
Levers
This section illustrates linkage mechanisms which can he adapted as hand
or loot levers. The materials used are round solid shaft and flat mild steel
bar. The bar is rut with concave curves at the ends to accommodate the round
solid sections which form hearings and pivoting points. Fig. 12 is an example
of the use of the trammel bar where the kerf is included when setting the
radius, and the measurement should, therefore, be taken from the edge of
the oxygen cutting jet which is farthest from the trammel bar pivot point.
The importance of adequately filling the craters at the weld ends is again
stressed as it is at this point that the maximum strain arises. The old adage:
'a chain is only as strong as its weakest link' can he adapted to welding
in the form: 'a weld is only as strong as its weakest crater'.
The foot pedal itself' can either he cut from chequer plate. if available, or the cross hatchings to give a good anti-slip grip can he made by welding heads deposited with a low amperage to give a high build-up.
Pitman Arm for Binder Knotter
The original part is shown on the left in Fig. 13. The centre example is
in the tack-welded condition. It is made from three short sections of round
solid, one long and two short lengths of smaller diameter round section.
To obtain the reduced diameter of the longest part of this pitman arm it
has been forged down.
Alternatively two different diameter sections can be used and butt welded together: this is a very useful method where a forge is not available. This forged down section is threaded to take the adjusting nuts and again the use of hand dies or screwing machine can he dispensed with, often resulting in a saving of production costs, by using a long bolt with the hexagon head cut off. The final state of the fabrication is shown on the right with all welds completed, hearing holes drilled and locking nuts fitted.
An alternative method of fabrication is shown in Fig 14. This illustrates the way in which fabrication can produce the desired result without making an exact copy of the original part. The yoke A provides a stronger structure than the original malleable casting and is easier to make and assemble. It also reduces the number of parts by one.
Connecting Rods
The various examples of this type of fabrication introduce channel and R.S.J. sections
in addition to flat, square and round bar, see Fig. 15. The initial sketch
shows the forged original: none of the replacements are precise copies of
the original.
It should he emphasised that there is rarely any need for fabrications to copy slavishly cast or forged originals and usually, if they do, the result is a waste of steel, undue strength and weight in the fabrication and a great deal of unnecessary preparation to produce a replica.
Cranks
This example shows a typical single throw crank. Four different methods of
welding the round shaft sections to the mild steel throws are shown in
Fig. 16. The subsequent use to which the crank is to he put will decide
which you will employ.
A The countersunk hole should leave a small land, say 3/32 in., being part of the original drilled hole. This prevents burning through during welding and at the same time allows a high amperage to he used to ensure adequate root penetration.
B This joint can he the weakest of the four shown unless adequate thickness of material is allowed in the throw. Given adequate substance at this point the joint is square at the junction of the round to the flat and the appearance is neat.
C This joint requires a good fit between the round and the flat. The drilled hole should be a driving fit for the round section, otherwise the 'work' that will develop will gradually crack the fillet weld. For exceptional strength the round can be threaded and the flat tapped and the parts tightly screwed together and the fillet weld then applied.
D This form of construction is tile simplest of the four. Its drawback is the intrusion of the fillet weld into the bearing area, but this can he overcome by fitting a thick washer of an internal diameter sufficient to accommodate both the round section and the fillet weld attaching it to the throw. The slight disadvantage of this form of construction is that the point of strain is carried further away from the throw and its leverage consequently increased.
Bell Crank Levers
These bell cranks introduce the use of pipe as an addition to the previous
sections used.
Key to Fig. 17
A This shows the use of pipe for making off-set bell cranks. The use of the trammel bar enters into the making of all three types of bell crank shown.
B This lever can be made from mild steel bar heated and forged in the tire. Alternatively if a quantity of these cranks are being, made the lever itself can be cut from plate and each 'nested' into the next to minimize scrap. In this example the bend is drilled to take the tubular hearing and the bearing is secured with fillet welds applied from each side.
C In this example a narrower bearing surface is needed and this is achieved by welding washers on either side of the bend. These can either he machine-made washers or discs cut by the trammel bar front plate or bar of the required thickness.
Castor Wheel Mountings
Material used is flat and round stock.
Key to Fig. 18
A This castor wheel mounting is made of material which it would be difficult to forge to shape, or where material of sufficient size is not available for the fork to be made in one piece. Fillet welding is used throughout. The vertical spindle can lie fillet welded both above and below the crown plate of the fork for extra strength.
B This fork has been made from a single piece. After being cut out by the cutting blowpipe it was formed by heating and bending by hammer or machine.
C This fork is designed to take very heavy loads. the reinforcing
web plates are cut from a length of mild steel bar. A rectangle is first
cut from the bar and then cut in half diagonally. This fork will withstand
great sideways thrust and can therefore he operated with safety on rough
and uneven ground. The vertical spindle should he double fillet welded above
and below the crown plate as in A.
