Discussion:
Help with Pivot Steer stability

hi,

we have a number of Pivot Steer (aislemaster/bendi) trucks and although operating them is fine i have some trouble with the stability characteristics, namely elevating with mast pivoted against straight.

training standards and best practice dictate that when elevating a load the mast should be in the pivoted position i.e. facing the side, but when lowering a load it should be straight i.e. facing forward.

what im trying to ask is what difference does it make? would the load not be just as stable going up or down the mast in either position?

cheers.
  • Posted 18 Dec 2017 22:33
  • Discussion started by TonytheTrainer
  • Aberdeenshire, United Kingdom
Showing items 1 - 5 of 5 results.
One of the problems trying to set the truck in terms of the stability triangle is that the normal arrangements don't apply. I tried it with two triangles, the joint apexes being on the pivot king pin. I am not sufficiently adept at force diagrams and complicated geometry, so I failed. This is what the late George Coates spent one Christmas doing, to the angst of his wife and family.
The instructor who carried out my conversion course, some years ago, claimed that he could only train on a Flexi, not on a Bendi, because they were different, until I explained that the only significant difference lay in which axle provided drive.
In the end, I completed the course using both types of truck, in two different locations over two days as one had gone unserviceable.
  • Posted 1 Feb 2018 21:44
  • Reply by Pusey
  • Somerset, United Kingdom
Just an observation on my part, re the stability of the Bendi truck. Best way to demonstrate stability of these machine is to pick up a load in an open area in the travel position. Steer from left lock to right lock lock, and observe the steering pivot point and how the load rotates around the steering pivot point, through 180 deg. The Bendi is at its most stable with the load straight ahead. But has been load tested with the load at 90 deg to the straight ahead position. I also found it difficult to understand the stability triangle theory on these types of machine, because of course when you carry out above the triangle also rotates through 180 deg at the back end of the machine and at both extremes is outside of the wheel base
  • Posted 31 Jan 2018 05:09
  • Modified 31 Jan 2018 05:14 by poster
  • Reply by BobbyT
  • Wilts, United Kingdom
thanks for the reply pusey.

so to summarise, you are saying that straightening the front end before lowering the load is less about maintaining stability and more to ensure clearance from the racking system?

this was my thought also.
  • Posted 15 Jan 2018 20:22
  • Reply by TonytheTrainer
  • Aberdeenshire, United Kingdom
When I worked for ITSSAR, I came across the pivot-steer truck for the first time and I tried to interpret the stability triangle theory to the truck, and expressed my doubts as to stability to George Coates, then Director General of BITA. He later claimed that I had ruined his Christmas as he played with two triangles of card on the dining room table to try to understand the issue.
The main problem is alignment of the fork arms when entering a load or picking a load from a rack cell, particularly at height. This is confused when normal instruction is that a truck should be positioned for a straight entry or load destack before using the hydraulics to maintain maximum stability, yet the pivot steer design means that steering is necessary in both stacking and de-stacking manoeuvres.
The perceived method for stacking with a pivot steer truck is to position the truck to the opposite side of the aisle, stop and rotate the mast in the aisle. Then, move forward and reduce the steering lock to maintain the load in a straight entry into the cell.
The reverse is true when de-stacking. Here, however, the fork arms enter the load and need to be steered in by reducing steering lock as you go in, straightening the truck. Once in position and with the load lifted, steering lock has to be reapplied as you extract the load. However, the problem is that the far corner of the load is invisible to the operator, and side shift might need to be applied to ensure that that outer corner of the load remains clear of the cell uprights. This effect may be magnified as rear tilt may be limited, particularly on higher-lift masts. The effect of this is to finish the extraction with the truck on full lock. To maintain maximum stability, the truck should be stopped while the steering is returned to the straight-ahead position. Once that has been completed, the load may then be lowered to a safe travelling position. This means that the load is turned when it is raised, but this may be necessary to ensure that the load can be lowered completely clear of any lower obstruction.
One aspect of stability on a pivot-steer truck is that the counterweight and battery pack are often heavier and set further back and lower than in a conventional counterbalanced truck, thereby increasing the counterbalance effect.
I have seen some scary moments with pivot-steer trucks, but I do have an operator certificate having received training on both main types available at the time (Bendi and Flexi). I was responsible for BITA classifying the truck as a separate type, for ITSSAR developing a separate test and I wrote the original RTITB Trainer's Guide. It is now several years since my operator's certificate was refreshed as I have no further need of it, being retired.
  • Posted 4 Jan 2018 21:01
  • Reply by Pusey
  • Somerset, United Kingdom
David
It should not make a difference. I think the reasoning is after a load is picked up from the rack, the operator needs to clear the rack before lowering. The truck should be just as stable lowering as it is lifting.
  • Posted 19 Dec 2017 00:28
  • Reply by BREWSKI
  • Nebraska, United States

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