ON TEST
All play and no work
This month our resident plant expert Jim Maccall looks at how to maintain value on our small plant. A boringsubject all round…
Small and medium-sized plant in the hire sector rarely suffers from over greasing by our customers and along with the ‘normal’ wear and tear you end up with an excavator that is less than a precision digging machine.
On small stuff, you can grab the digger bucket and wobble it about by hand with what seems like six inches of play and there is only 1,500 hours on the clock. Customers notice this and refer to you as running ‘owd dogs’, unlike your neighbouring hire firm who has shiny new kit. It’s not the end of the world until you come to move the machine on, or trade it in with resellers and dealers.
This starts with sucking of teeth and they knock at least £500 off its value. So before trade in we whack in new pins and bushes and depressingly this only takes out less than 50% of the movement, as a big chunk of the play is in the fixed ends of the pivots.
Boom to kingpost pin clearance is clear to see. What’s the cause?
There are three common causes and we often end up with a combination of all three.
Lack of maintenance/greasing All the pivots on an excavator, dumper or MEWP either have floating pins where there are bushes or bearings in all the bores, or most commonly one bit has a bearing with the pin retained in a fabrication or casting by clips, bolts or similar. As soon as that pivot goes dry, due to lack of greasing, the pin and bush end up grabbing hold of each other forcing the pin to rotate in the fixed bore and wears its way through it. If caught quickly it can be easy enough to resolve, but leave it a while and either the pin falls out and bends something or we end up with pins and bores wearing where no movement should take place.
46
Arduous applications Some jobs are hard on kit, add a breaker to any machine and essentially Newton’s third law does the damage. The machine is exerting enough force to break concrete and all that force is transmitted through every pivot. The bearing surfaces and fixed journal wear as a result where their clearance from new allows for tiny movements with every beat of the breaker. This is easy to spot as the bore ends up with ovality or lonzenged wear.
Sloppy pin retention fix - the pin can still move!
Movement built in From new, manufacturers work to tolerances where you need clearance between a pin and its bush or bearing, but you also need clearance between the pin and its fixed bore otherwise you wouldn’t be able to get the pin in. On a mini excavator, from the revolving frame to the digging bucket tooth are twenty different places where clearance is needed. So even if the manufacturer only allows 0.1mm at each, that’s still 2mm.
This would be rare and it’s more likely that you end up with total clearances of 5mm or more. Add in the geometry, and on a brand new machine you could see 20mm or more movement from new. We need clearance to allow the grease to get between the bush and pin but at fixed pivots it’s rarely helpful except when fitting and removing the pins. Some makers glue the pins into these places which is lovely until a mechanic has to remove a ram and finds the
A kingpost alone has eight different bores that can have excessive clearance or wear.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
