Mobile Elevating Work Platform – MEWP

Alessandro Zanin 1041177 – alessandro@alessandrozanin.it – Degree in Mechanical Engineering

View ITA version

 

Target

Demonstrate the feasibility performances as declared regarding the project PRT_26 from viewpoint of dynamics:

  • Duty cycle complete with combined movements
  • Reactions to the ground
  • Couple of mean stress of the thrust bearing operated on performance of maximum opening
  • Frequencies its components (arms in particular)

In all the simulations carried out and in the related video will be represented by the platform components that meet the characteristics of mass (and their centers of mass) but not of the form given that innovation lies precisely in their geometry (the drawings are not those of the aerial part analysis of the platform).

As best shown later will be applied very high accelerations and decelerations to verify the stability even in the event of failure of the machine.

The comments on the results will be displayed after the description of PRT_26 but now I want to dedicate these results to all ITALIAN companies, industry leaders, that despite being built and currently in circulation (IN GERMANY) machines that mount the chassis mm7.7 of the PRT_26 have judged impossible the construction of a machine with such performance (in particular, I quote the words of a technical director of a corporation): “If we have failed, that we make platforms since thirty years, to make a machine like this, he wants to be could you? “

The following is a brief description about the project PRT_26.

 

Introduction

In 2009 I designed a new concept chassis, Mercedes Sprinter chassis applied to [wheelbase 4325 mm], VW Crafter [4325 mm], Iveco Daily [4100 mm] 3.5 t (B license).

From this was born the overall design of the platform.

  • Performance of maximum:

    • Weight: 3400 kg (full tank)
    • Weight distribution: Front axle 1630 kg;

      Rear axle 1770 kg;

    • Carpentry thickness: ≥ 3mm
    • Work Area: Appendix 1.1

 

Frame (base)

The design meets the standards required by the UNI EN 280 of August 2009, and particular attention was paid to the creation of special easy to assemble (carpentry).

The manufacturing steps of the first model were performed under the supervision of the undersigned;

the operating procedures have led to the installation and immediate enforcement of the same on the chassis of the van; was therefore not necessary to build a second specimen.

On this foundation were installed aerial part components already existing in possession of the customer.

 

Aerial part

The study of all the components above the thrust bearing meet the standards required by the UNI EN 280 of August 2009.

In particular, some of these components (TURRET) allow for the reduction of weight and the increase of the stability and rigidity of the structure.

 

Load tests (virtual and real)

The 3D modeling and subsequent analysis with FINITE ELEMENT METHOD (FEM) has allowed the evaluation of the assumptions made for the new structure.

With these tools have occurred stress, strain, joints, fatigue (life of the product) and buckling (structural instability).

Structural performance (virtual version) frame:

  • Axial load thrust bearing = 210% of rated load
  • Moment (torque) bending thrust bearing = 120% of rated torque obtained in the fully extended position (Annex 1.1) of the machine (front, rear, side)
  • Weight of truck

With the above-mentioned loads the structure (base) has a life in excess of 6×104 cycles and a deformation of less than 10mm in the position of maximum stress (maximum horizontal extension arms aligned with the front leg of the base).

Structural performance (real version) frame:

  • Axial load thrust bearing = 210% of rated load
  • Moment (torque) bending thrust bearing = 120% of rated torque obtained in the fully extended position (Annex 1.1) of the machine (front, rear, side)
  • Base mounted on chassis Mercedes Sprinter 313 CDI [wheelbase 4325mm]

Tests have confirmed the results of virtual to 100%, in particular a calculated deformation of 7.7 mm was measured feedback of 8mm.

Overturning: success.

 

Dynamic simulations

From here we start moving the machine observing the maximum speed allowed by law but in fact what we want is it accelerating and braking abruptly… bring it up to the limit!

It is trivial to note that the acceleration in the involved are unacceptable for an operator who is working on the basket, but here we are talking about pure performance.

Basket accelerationIn the graph can be seen around 280 [sec] of the peaks due to a deceleration extreme in the stage of maximum opening of the piston which raises the arms. Beyond this event, there are accelerations and decelerations in excess of 16 [m/s2] without the machine hints at tilt.

 

The law provides for maximum speed, respectively:

  • 0.4 [m / s] for the movement of the boom
  • 0.4 [m / s] velocity for lifting and lowering device
  • 0.7 [m / s] peripheral speed due to rotation

Peripheral speeds during ascent-descent and rotation

 

We see the commands that require these movements:

 

Command arms

Command lift

Command rotation

The consequences of these dynamic stress and its ability to endure the machine are well illustrated by the next graph which maps the evolution of the normal forces that results on each stabilizer:

 

Each curve (green, red, yellow and blue) shows the reaction force of each vertical stabilizer respectively. It is precisely this graph which justifies the stability of the machine; we can notice that there are times when the reaction of A stabilizer goes to ZERO. It is not surprising, since we are working to peak performance and the point of contact between the stabilizer and ground is a ball (even constructively feet have a ball joint); this, coupled to the deformations of the structure, bring (under certain conditions) to have a leg that does not contribute to equilibrium. Little bad, we know that a plane is defined by three points in space, and then, as shown by the dashed bands, at each moment at least three on four stabilizers offer reaction and the balance is guaranteed.
It goes without saying that the static dimensioning of the stabilizers must take this into account.

 

TORQUE item Thrust bearing

The law that sets standards for the design of these machines give an indication for the calculation of the life of the platform in very general terms:

  • Normal duty: 10 years, 40 weeks a year, 20h per week, 5 cycles of loading per h;
  • Heavy duty: 10 years, 50 weeks a year, 40h per week, 5 cycles of loading per h.

 

 

Well, if we consider the cycle analyzed in this simulation (which is a cycle at peak performance! thing that never occurs in reality), the torque that acts on the thrust bearing is an average of 38170 [N ∙ m], which means that for a FEA executed in favor of security, is to use this value as the peak load for the FRAME element, not just, even the trend of the stress load will influence FEA analysis.

Explanation

FEA analysis we can set the type of stress cycle is alternating with a ratio R = -1, alternating with zero-based ratio R = 0, etc…

Having a ΔTorque of 38000 [N ∙ m] with stress cycle zero-based, meaning that the peak is indeed 38000 [N ∙ m]. Therefore, by referring to the law, for a normal use of the machine FEA must give positive results with this value and exceed the 40000 cycles.

At this point we understand the importance of knowing this value because you can verify the safety against fatigue that the machine offers.

And this was one of the points that gave reason to the FEA previously made, because the FRAME named MM7.7STRUCTURE (available on the site www.alessandrozanin.it) gave positive results with the following request:

  • Axial load thrust bearing = 210% of rated load
  • Moment (torque) bending thrust bearing = 130% of rated torque obtained in the fully extended position (Annex 1.1) of the machine (front, rear, side)
  • Weight of truck

Say 130% of rated torque means you have loaded the frame with a torque of 50000 [N ∙ m] in the most unfavorable position (constructively weaker) and have passed the limit imposed by law, exceeding 60000 cycles.
By the way: for the rated torque are considered 150 [kg] on the basket work because the EN280 to requires static tests and rollover with 50% more weight than the permitted working of the machine; business performance are those given in Annex 1.1.

 

Frequencies of the critical components

Finally here because it is important to have an order of magnitude of those that are the own frequency of vibration of the machine.
In particular the elements affected by the danger of forcing imprinted with a certain pulse are the arms that when extended present appreciable deformation.

We believe in the movement of descent or ascent and imagine that for an electro-mechanical problem or for fun / unconsciousness of the operator the machine to start functioning in spurts.

This condition can become very dangerous and create resonance conditions that more than having dangerous effects on the structure can lead to amplification of the loads way that it produces the machine overturning.

Although this analysis must be done with due caution in order not to be misled neglecting some situations and assuming that the first natural frequency is the most dangerous.

As best shown below is necessary to evaluate a “band” of frequencies to be avoided dictated by different load conditions, in particular:

  • Without operator on board

 

  • With operator on board

 

Watching the video you can see immediately that the natural frequencies most dangerous are the first two because they are the lowest and those most likely to manifest itself.
It also explains why the “band” of frequencies: if you had only analyzed the case with operator, the beneficial effect of the weight of the same operator and related equipment that brings with it would outweigh the lowest frequencies that can occur when maneuvering the machine from the ground or when the weight in the basket is lower than that used for the analysis of resonance.

It is relevant the frequency difference between a basket loaded 180Kg compared to an empty basket:

  • Empty basket: 4.38 [Hz]
  • Full basket : 21 [Hz]

Attention to control signals and control of proportional valves!

A final compiled analysis with ANSYS ® on FRAME showed the vibration modes are in contrast to the aerial structure with frequency of:

  • 12.7 [Hz]
  • 27.6 [Hz]

which encourages the mutual vibration damping of the structures above and below the thrust bearing.

 

Benchmark

Two words on competition are essential, and will explain the reason for the birth of this project.
Currently only one company has succeeded in the manufacture of a machine with performance similar to that contained in this brief dossier.
The problems that are analyzed here are of course only a part of the complex that characterizes the project but has served to complete so formal and scientific and removing the last doubts about the ability to succeed the project same.

Recalling the maximum performance mentioned at the beginning:

  1. Weight: 3400 kg (full tank)
  2. Weight distribution: Front axle 1630 kg;

    Rear axle 1770 kg;

  3. Carpentry thickness: ≥ 3mm
  4. Workspace: Appendix 1.1

points a) & c) are those that make a difference to the Ruthmann TB270.

Ruthmann offers its services on a Chassis with tare 3.5 [Ton] mounting some arms with thickness below 2 [mm], a turret from the high flexibility and a thrust bearing by weight (the thrust bearing of PRT_26 weighs 110 [kg]! ).

Ruthmann is an industry leader in platforms and the machine that have created show it; in Italy instead:”If we have failed, that we make platforms since thirty years, to make a machine like this, he wants to be could you? “

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