gmi machine tipping standard - general...

Portable Machine TippingDesign Standard

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Portable Machine TippingThis standard contains design criteria and specifications used in the safe design of generally portable equipment to greatly reduce the risk of tip hazards.

The limit of this document is pieces of equipment that are securely fastened to their surrounding facility. In that case they are not required to adhere to this standard. However, those specific pieces of equipment must have associated documentation explaining the appropriate rigging and anchoring methods.

Table of Contents1. Executive Summary ............. 1

2. Introduction .......................... 2

3. Modes of Failure .................. 3

4. Machine Design Criteria Intro 5

5. Machine Tipping Calculator . 6

6. Maximum Angle of Inclination 9

7. Horizontal Force ................ 10

8. Sudden stoppage .............. 12

9. Appendix ........................... 13

10. Revision Summaries .......... 18

1. Executive SummaryThis portion of the document summarizes the critical machine requirements. If needed, the Machine Tipping calculator can be found on page 7. The supporting information for all calculations and rational is contained in the following pages.

All machines must be able to withstand all three of the following:

A maximum angle of inclination of 15 Degrees.

A force equal to 30% of the machine weight (up to a maximum force of 120 lbs) applied horizontally at the top of the machine (up to a maximum height of 62”) evaluated on a sloped floor with angle of 2.4 degrees.

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A sudden stop from a speed of 47 in/sec.

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2. Introduction

Any piece of equipment is susceptible to unintended overturning or tipping, which occurs when the Center of Gravity (CG) moves outside the support base perimeter. In the case of a four-legged frame design, that base is a rectangle where the points of it are the contacts between each caster and the ground. Alternatively, in the case of a three-legged frame design, the base is a triangle, again with the points at the contact point between the caster and the ground.

Figure 1 – Support Base Perimeter.

This risk is most often elevated with portable equipment since permanently installed equipment is usually anchored to the floor, other equipment, or held in place by utility connections.

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3. Modes of Failure

In this analysis, a mode of failure is a manner in which any machine or equipment is caused to tip over. In general, unintended machine tipping can be caused by one (or any combination) of several modes of failure. This analysis will consider three of them, which are tipping due to:

1. Maximum Angle of Inclination

Every machine should be able to withstand a certain angle of inclination without tipping over. This scenario may occur on a loading ramp or during uneven lifting.

This mode is specifically different than the next mode in that in this case the machine starts at this angle and needs to remain upright. The next failure mode assumes the machine starts level or at a much smaller level.

2. Horizontal Force

This failure mode entails a machine tipping over when excessive horizontal force is applied relatively high on the side of the machine. This may occur when attempting to move a portable piece of equipment and the casters meet one or more of the following conditions:

Locked

Not turned in the appropriate direction.

Pushed up against something, like a curb or another piece of equipment.

It could also occur if a machine is on legs instead of casters and they don’t slide as easily as intended.

If pushed far enough, the CG will move over and outside the support base perimeter. Once that occurs, gravity will pull the machine all the way over.

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3. Sudden stoppage

The final failure mode considered in this analysis occurs when a person is pushing a portable machine near typical walking speeds and the caster support base is suddenly stopped. This could be caused by a caster hitting a piece of floor debris, a curb, another machine, or a gap in the floor (eg. those outside of elevators and loading docks).

In these scenarios, the force required to move the CG beyond the support base perimeter comes from the deceleration of the support base and the remaining inertia acting on the CG. If there is enough inertia, the CG will move beyond the support base perimeter and will continue to tip all the way over.

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4. Machine Design Criteria Intro

Each failure mode necessitates measurable criteria to aid in design so that a machine can be evaluated in terms of tip hazard risk. Additionally, Non-General Mills equipment brought into our facilities must also be evaluated for this risk.

The following paragraphs discuss the quantitative metrics and values for each failure mode to be used in evaluating compliance with this standard.

A design calculator is included in the next chapter to aid in determining compliance. All of the formulas used in the calculator are included in the Appendix at the end of this document.

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5. Machine Tipping Calculator

The attached Machine Tipping Calculator (embedded below) is provided to aid in determining if a given machine is in compliance with this standard.

It is designed to be simple to use, only requiring four input values. They are shown below in the red box.

Figure 2 – Machine Tipping Calculator.

The results are listed above in the green box in the form of an answer to the question, “Is my machine compliant for these failure modes?” The result for each one shows up as a green “Yes” or red “No”, as shown in the green box. It is very possible for some to be “Yes” and some to be “No.” All three must be “Yes” for the machine to be compliant with this standard.

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The blue box includes calculated values so the reader can evaluate for him or herself the implied risk as compared to the standard.

The input values needed to type into this calculator are easy to find if a solid model exists of the complete machine design. The CG information can be used by accessing the mass properties of the assembly model. In SolidWorks, the mass properties can be found by clicking on the tool bar Tools / Mass Properties.

Figure 3 – Mass properties.

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The CG location can be viewed mathematically under the “Center of Mass” heading in the Mass Properties dialogue box, which is in relation to the assembly / part origin. The CG can also visually be located in the model, represented by the purple triad, as shown above circled in red. The weight (mass) can also be discovered from this dialogue box above under the “Mass” heading.

Note that certain solid model components do not always have accurate masses assigned to them. In order for the reported mass of a solid model to be accurate, each part needs to have the proper (or approximate) material type assigned. All downloaded components (gearboxes, motors, ultrasonic transducers, etc.) must either be accurate in terms of weight and geometry, or have a mass assigned to them.

If a model does not exist, these four measurements need to be taken manually. Machine height and weight will not be difficult to measure assuming a measure and a scale are available.

However, the CG location will be more difficult to attain. If precision is not required, it is reasonable to assume that the CG is in the geometric center of the machine. Therefore, the machine height can be halved to attain the value “a.” Similarly, the base width measurement can be halved in order to attain the value “b”. If it is obvious that the machine is lopsided in any direction, the estimated CG can be shifted a reasonable amount to accommodate.

If precision is required, the two options are to try to model the machine in solid-modeling CAD software, or to use a system of scales to determine how much weight is placed on each leg. Then, the percentage of the total weight that each leg carries represents the percentage of the total machine length that the CG is from the light end of the machine in the respective direction.

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6. Maximum Angle of Inclination

All machines should be able to withstand a maximum angle of inclination of 15 Deg. This value was chosen based historical rules of thumb used in the JFB Machine Shop.

It should be noted that standards within CE only require that a machine withstand a slope of 10 degrees. This implies that this standard is slightly more conservative than the CE requirements.

In the event that the design does not allow the machine to remain upright when set on an inclination of 15 degrees, remediation should occur to bring the design into compliance. Usually this means widening the base since the machine height and CG are determined by machine function and can rarely be substantially adjusted.

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7. Horizontal Force

Any machine should be able to withstand a force equal to 30% of the machine weight up to a maximum force of 120 lbs (or up to a machine weight of 400 lbs) applied horizontally at the top of the machine, with a 62” max height. This should also be evaluated in the scenario that the machine is sitting on a sloped floor with slope angle equal to 2.4 degrees (0.5” rise over 12” run) in any direction relative to the machine.

In the event that the design does not allow the machine to remain upright when subjected to the give force, remediation should occur to bring the design into compliance. Usually this means widening the base since the machine height and CG are determined by machine function and can rarely be substantially adjusted.

30% was chosen by evaluating multiple pieces of equipment on their weight, base width, height, and their center of gravity (CG) position. 30% of the machine weight represents an approximate amount of force that a person might push with before stopping to evaluate the cause of immobility.

The height of 62” was chosen since the 95% American male is 75” tall. Therefore the related shoulder height is about 62” above grade.

The value of 120 lbs was chosen since that is an approximate maximum force that any single person would be able to momentarily apply to the side of a machine at the height of 62”. Above that level, the risk of someone pushing with a force equal to 30% of the machine weight tapers dramatically.

Note that a force in terms of the machine weight is used instead of a “sufficiently large” fixed force value. Although for large machines this would work fine, for smaller devices a “sufficiently large” force value may be overkill and extremely unpractical.

Consider a plastic pushcart with some small piece of lab equipment on top. The whole assembly may weigh only 30 lbs. If subjected to a requirement mandating it withstand 120 lbs of lateral force at the top of the machine, it would quickly tip over. To meet the requirement it would need to have an unreasonably large base, or a large (and unnecessary) weight added to the bottom.

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Instead, a user has the tendency to subconsciously evaluate the weight of a machine when attempting to move it. They then initially apply the appropriate amount of force. After pushing and subconsciously evaluating if more or less force is required, the user adjusts accordingly.

Using the previous example, a user is not going to push with 120 lbs of force on a small plastic push cart with a small device on top. Instead he/she will stop well below that force level and try to identify the cause of immobility. Therefore, the requirement that a machine be able to withstand horizontal forces equal to percentage of its weight is more appropriate.

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8. Sudden stoppage

Every machine must be able to withstand a sudden caster stop from a speed of 47 in/sec such that the CG starts to rotate up over the closest edge of the support base perimeter.

The value of 47 in/sec was chosen by evaluating multiple pieces of equipment on their weight, base width, height, and their center of gravity (CG) position. 47 in/sec represents the average velocity that the “tippy” machines (those evaluated to be tippy upon inspection) were calculated to tip at or below when subjected to a sudden stop.

The machines that were deemed to be “stable” upon inspection were calculated to tip only at a higher moving speed than 47 in/sec on average.

The calculation is based on the machines initial kinetic energy, the geometry of the base and CG, and the fraction of kinetic energy that is converted into potential energy.

Assuming some supports lose contact with the ground during the sudden stop, the machine is considered to meet the requirement if it does not tip over and instead comes back to rest on its base.

In the event that the design does not allow the machine to remain upright when subjected to a sudden stoppage, remediation should occur to bring the design into compliance. Usually this means widening the base since the machine height and CG are determined by machine function and can rarely be substantially adjusted.

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9. Appendix

This chapter will address a number of items that are not generally fit or relevant to the average reader. However, they are provided here to address specific questions that may arise relating to the enclosed content.

1. Horizontal Force Formulas

We begin with the four values measured from the machine of interest. They are height of CG (a), horizontal distance from CG to closest support base edge (b), Weight (W), and height to top of machine with max of 62” (H).

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Figure 4 – CG Geometry.


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From those values we can calculate the angle (θ) formed between the relevant support edge, a horizontal plane, and the CG. See above for illustration.

∅=tan−1( ab ) (Eq. 1)

It should be noted that the standard indicates that this test scenario should be considered with the floor already containing a 2.4 Deg slope in the direction that the machine might most easily tip. Therefore, add 2.4 Deg to θ at this point. θ becomes θs.

∅ s=∅+2.4 (Eq. 2)

Next we will calculate the force (Fcg) required to tip the machine when applied at the CG height (a), and then the force (Ft) required to tip when applied at the top of the machine (H) (with a maximum of 62” high).

F cg=w

tan(∅ s) (Eq. 3)

F t=Fcg( aH ) (Eq. 4)

Now we can convert that force value into a normalized value as a percent (P) of machine weight that can be compared against a set limit that will apply for many different sized machines.

P=f tW

(Eq. 5)

If P is greater than the standard (30% for machines weighing less than 400 lbs), then that means the amount of force required to tip the machine over is greater than 30% of the machine weight. It is therefore compliant with the standard on this failure mode.

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2. Maximum Angle of Inclination Formulas

This is the Angle of Inclination (β) that will cause the machine to tip, even when at rest. It is easily calculated assuming (Eq. 1 has already been solved.

β=90−∅ (Eq. 6)

If β is less than 15 degrees, then the machine is not in compliance.

3. Sudden Stoppage Formulas

This calculation determines the max speed a machine may travel and still not tip all the way over given sudden stoppage.

Begin with setting the initial Kinetic Energy (Ek) of the moving machine equal to the Potential Energy (Ep) needed to raise the CG up to the tipping point, a distance of (c). (Some of the Kinetic Energy will be lost at this transition, but that will be accounted for later.)

EK=12mY 2 (Eq. 7)

Figure 5 – CG position when tipping.

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Where (m) equals the machine mass and (Y) equals the velocity in the initial direction of travel directly after stopping. Next,

EP=(12 ¿ft )mgc (Eq. 8)

Where (g) is the gravitational constant 32 ft/s2, and (c) is the distance (in inches) that the CG moves upward vertically when tipping. Now setting (Eq. 7 equal to (Eq. 8 yields

12mY 2=(12 ¿

ft )mgc (Eq. 9)

Simplifying and solving for Y gives

Y=√(2 ) ¿¿ (Eq. 10)

To find the initial horizontal velocity (V) of the machine that would cause the machine to tip and reach the limit of tipping over we solve the following equation for V.

V= Ysin (∅ )

(Eq. 11)

Note, this is where some of the energy (in this case represented by velocity) is lost.

Comparing this value (V) to the standard (47 in/sec) yields the

compliance/noncompliance result. If V is less than 47 in/sec then the machine is

not in compliance with the standard. Actions should be taken to remedy the

problem, most often widening the base of the machine.

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4. Other Failure Modes

There are many other ways in which a machine could tip over. The modes included in this standard were selected due to their common occurrence. To cover every scenario would require volumes of discussion and data.

The following list is a sample of other failure modes, but is not comprehensive. Other failure modes may include (but are not limited to)

Overhung loads (cantilevered)

o One criteria used in this scenario is to require the machine to withstand the weight of a large person (~300 lbs) hanging from the cantilevered point without tipping over. This criterion is probably not suited for all applications of this sort.

Uneven lifting

One leg or wheel falling into a floor drain

Extreme shaking or vibration

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10. Revision SummariesEnter information in these tables to track revisions to the master document.

NOTE: In the Entry column, enter your information between the dashes. The entries automatically update matching document properties.

Document Property Entry

Master Owner Zac Harder Increment revision numbers by one.

Master Revision Number 0

Master Revision Date 9/12/2013

Revision Number

Revision Date

Revision Owner

Revision Summary(Brief description of major changes)

0 9/12/2013 Zac Harder Original Development

1 9/16/2013 Zac Harder Requirement lowered from 150 lbs to 120 lbs Per Scott Tolson.

2 9/23/2013 Zac Harder Added Executive Summary on Page 1.

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gmi machine tipping standard - general...

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