Calculating horizontal load on a scissor lift
June 8, 2020 10:11 AM Subscribe
How do I calculate the horizontal force applied to the rail of a scissor lift if someone attached to the rated attach point were to fall out?
An attach point for fall protection must be rated 5000 lbs. The horizontal force necessary to tip over a scissor lift is usually between 100-200 lbs. If the impact of the fall equals, say, 3500 lbs, and the railing is 32" high, is there a way to compute how heavy the person would have to be to exceed the horizontal weight rating of the lift?
An attach point for fall protection must be rated 5000 lbs. The horizontal force necessary to tip over a scissor lift is usually between 100-200 lbs. If the impact of the fall equals, say, 3500 lbs, and the railing is 32" high, is there a way to compute how heavy the person would have to be to exceed the horizontal weight rating of the lift?
Response by poster: I'm not estimating it for practical purposes. I want to explain to a supervisor that a lanyard attached to a scissor lift's tie-off point isn't an acceptable fall protection anchor point. I've watched enough safety videos to know this is the case, but I was hoping to be able to prove the concept with some math. Maybe I'm out of luck.
posted by jwhite1979 at 10:39 AM on June 8, 2020
posted by jwhite1979 at 10:39 AM on June 8, 2020
Stick around to see if a heavy-equipment person can help.
posted by j_curiouser at 10:42 AM on June 8, 2020
posted by j_curiouser at 10:42 AM on June 8, 2020
DISCLAIMER: this is an interesting thought experiment, but get a competent engineer to work through this if you're considering doing it.
If I understand the problem correctly, you're imagining someone falling from the lift while attached to a particular point. Once the person's distance from that attachment point exceeds the distance of their safety line, it goes taut, applying a brief 3500-lb force to the person parallel to its length, and cancelling out the person's velocity in that direction.
The horizontal force applied to the person when the line goes taut will be the same as the force the line applies to the attachment point. Thus, you need the horizontal component of the 3500–lb force to be less than 100–200 lb. Taking the more conservative bound, this means that the line can be no more than sin–1(100 / 3500) ≈ 1.64° from the vertical when it goes taut. Note that if we assume the force of the impact is known, then we don't need to know the weight of the person to figure out the horizontal component of the force on the lift.
That's about all I can say with certainty with the information you've provided. The angle the line makes with the vertical when it goes taut is going to depend on several factors, such as where the attachment point is and how fast the person was moving horizontally when they lost contact with the scissor lift (i.e., when they fell off). But the bound above suggests that pretty much any horizontal motion would be problematic, as would any significant horizontal distance between where the person fell off and the attachment point.
Finally, note that a 3500-lb force will cause the body of a 175-lb person to briefly experience 20g, which is uncomfortably close to what a human body can survive and is well above the sort of thing that will cause serious injury. If you fall of the scissor lift and you're brought to rest with such a major force, whether the scissor lift subsequently falls over may be the least of your concerns. I suspect that most safety cables will apply less force than this, and stretch out over a longer distance to reduce the shock. A more realistic model would use the elasticity of the attachment cord to get an estimate of the force involved (this will depend on the person's mass); and then apply the above logic, using the known properties of the geometry.
posted by Johnny Assay at 10:44 AM on June 8, 2020 [4 favorites]
If I understand the problem correctly, you're imagining someone falling from the lift while attached to a particular point. Once the person's distance from that attachment point exceeds the distance of their safety line, it goes taut, applying a brief 3500-lb force to the person parallel to its length, and cancelling out the person's velocity in that direction.
The horizontal force applied to the person when the line goes taut will be the same as the force the line applies to the attachment point. Thus, you need the horizontal component of the 3500–lb force to be less than 100–200 lb. Taking the more conservative bound, this means that the line can be no more than sin–1(100 / 3500) ≈ 1.64° from the vertical when it goes taut. Note that if we assume the force of the impact is known, then we don't need to know the weight of the person to figure out the horizontal component of the force on the lift.
That's about all I can say with certainty with the information you've provided. The angle the line makes with the vertical when it goes taut is going to depend on several factors, such as where the attachment point is and how fast the person was moving horizontally when they lost contact with the scissor lift (i.e., when they fell off). But the bound above suggests that pretty much any horizontal motion would be problematic, as would any significant horizontal distance between where the person fell off and the attachment point.
Finally, note that a 3500-lb force will cause the body of a 175-lb person to briefly experience 20g, which is uncomfortably close to what a human body can survive and is well above the sort of thing that will cause serious injury. If you fall of the scissor lift and you're brought to rest with such a major force, whether the scissor lift subsequently falls over may be the least of your concerns. I suspect that most safety cables will apply less force than this, and stretch out over a longer distance to reduce the shock. A more realistic model would use the elasticity of the attachment cord to get an estimate of the force involved (this will depend on the person's mass); and then apply the above logic, using the known properties of the geometry.
posted by Johnny Assay at 10:44 AM on June 8, 2020 [4 favorites]
OSHA doesn't require fall protection on a scissor lift. It is considered a scaffold due to the railing set on it and its ability only to move vertically. Any lateral movement (driving) is to be done while in the lowest position, therefore fall protection is not required.
If you are performing any operation where falling out of the machine is a potential, you are not using the scissor lift in its original capacity.
And if you're concerned about tipping over, well let's just say you're probably not going to want to be tethered to that thing, better to have the ability to jump free, rather than getting pinned under it.
[W]hen working from an elevated scissor lift (ANSI A92.6 series), a worker need only be protected from falling by a properly designed and maintained guardrail system. However, if the guardrail system is less than adequate, or the worker leaves the safety of the work platform, an additional fall protection device would be required.
TL;DR Don't use any fall protection in a scissor lift, it's not OSHA required and is more of a liability than anything else.
Editted to add OSHA snippet.
posted by wile e at 12:15 PM on June 8, 2020 [6 favorites]
If you are performing any operation where falling out of the machine is a potential, you are not using the scissor lift in its original capacity.
And if you're concerned about tipping over, well let's just say you're probably not going to want to be tethered to that thing, better to have the ability to jump free, rather than getting pinned under it.
[W]hen working from an elevated scissor lift (ANSI A92.6 series), a worker need only be protected from falling by a properly designed and maintained guardrail system. However, if the guardrail system is less than adequate, or the worker leaves the safety of the work platform, an additional fall protection device would be required.
TL;DR Don't use any fall protection in a scissor lift, it's not OSHA required and is more of a liability than anything else.
Editted to add OSHA snippet.
posted by wile e at 12:15 PM on June 8, 2020 [6 favorites]
There are several variables that you need to take into account in this scenario.
Note: I'm not a scissor lift expert, but I've worked with (and on) one, a JLG 330LRT for several months.
First: the force you need to apply to the lift to tip it over depends on how wide it is and whether its jacks are extended if it has them, and how high the effective centre of gravity is when extended maximally (so how much mass is still 'down there' in the engine and ballasts). The force specified is supposed to be acting horizontally, and sideways.
100..200lbf sounds like you're dealing with a much smaller unit than the one I worked on.
Now, a person dropping off the platform will be falling more or less downwards, so most of the force exerted on the lift platform by the fall arrestor gear will not be in the direction of the 'maximum tippability' force but perpendicular to it. In addition to that you can't readily convert a person's mass into the force acting on the attachment point, as the arrestor gear will limit the deceleration that person will undergo as a function of the stretch distance implemented in the harness. Note that a harness for working on a platform is supposed to have a short lanyard, only long enough that a person can move about maybe their own body length. This also limits the height a person can drop, and with it limiting their maximum vertical speed. Which again means less deceleration to come to a stop.
Tl;dr: if you need calculations for supporting your point against your supervisor you need to look at more than just the tipping force and the railing height.
posted by Stoneshop at 12:47 PM on June 8, 2020 [1 favorite]
Note: I'm not a scissor lift expert, but I've worked with (and on) one, a JLG 330LRT for several months.
First: the force you need to apply to the lift to tip it over depends on how wide it is and whether its jacks are extended if it has them, and how high the effective centre of gravity is when extended maximally (so how much mass is still 'down there' in the engine and ballasts). The force specified is supposed to be acting horizontally, and sideways.
100..200lbf sounds like you're dealing with a much smaller unit than the one I worked on.
Now, a person dropping off the platform will be falling more or less downwards, so most of the force exerted on the lift platform by the fall arrestor gear will not be in the direction of the 'maximum tippability' force but perpendicular to it. In addition to that you can't readily convert a person's mass into the force acting on the attachment point, as the arrestor gear will limit the deceleration that person will undergo as a function of the stretch distance implemented in the harness. Note that a harness for working on a platform is supposed to have a short lanyard, only long enough that a person can move about maybe their own body length. This also limits the height a person can drop, and with it limiting their maximum vertical speed. Which again means less deceleration to come to a stop.
Tl;dr: if you need calculations for supporting your point against your supervisor you need to look at more than just the tipping force and the railing height.
posted by Stoneshop at 12:47 PM on June 8, 2020 [1 favorite]
Response by poster: TL;DR Don't use any fall protection in a scissor lift, it's not OSHA required and is more of a liability than anything else.
Absolutely. This is 100% the point I want to make to my supervisors, and I was hoping to have the math to show the likelihood of the lift falling on someone tied off to an anchor point. We currently have a fall protection requirement on scissor lifts, and we spend money hand over fist for harnesses and lanyards when what we really should have is a simple belt restraint to keep people from climbing or reaching over the rails.
posted by jwhite1979 at 12:50 PM on June 8, 2020
Absolutely. This is 100% the point I want to make to my supervisors, and I was hoping to have the math to show the likelihood of the lift falling on someone tied off to an anchor point. We currently have a fall protection requirement on scissor lifts, and we spend money hand over fist for harnesses and lanyards when what we really should have is a simple belt restraint to keep people from climbing or reaching over the rails.
posted by jwhite1979 at 12:50 PM on June 8, 2020
Response by poster: 100..200lbf sounds like you're dealing with a much smaller unit than the one I worked on.
The 330LRT has a 2250 lb platform rating, and the models I usually deal with have a 500 lb platform rating, so yes, there is a big difference there.
n addition to that you can't readily convert a person's mass into the force acting on the attachment point, as the arrestor gear will limit the deceleration that person will undergo as a function of the stretch distance implemented in the harness.
There's a fall protection video on YouTube that talks a bit about this. It shows how without shock absorption, a person 220 lb person falling 6' will experience 4,000+ lbs of arresting force, but that by using a shock absorbing lanyard, that number is reduced to 832 lbs. I get a little confused here because reducing the arresting force doesn't mean you're allowed to reduce the strength of your anchor point; it still has to be rated for 5,000 lbs. I've assumed that means the anchor point doesn't undergo the same shock absorption as the person falling? I was a liberal arts guy, so the is a gray area for me.
Now, a person dropping off the platform will be falling more or less downwards, so most of the force exerted on the lift platform by the fall arrestor gear will not be in the direction of the 'maximum tippability' force but perpendicular to it.
Is the takeaway, then, that a scissor lift that could tip at 150+ lbs of horizontal force would not likely receive enough horizontal force to tip over if a person tied off inside were to fall over the rail?
posted by jwhite1979 at 1:15 PM on June 8, 2020
The 330LRT has a 2250 lb platform rating, and the models I usually deal with have a 500 lb platform rating, so yes, there is a big difference there.
n addition to that you can't readily convert a person's mass into the force acting on the attachment point, as the arrestor gear will limit the deceleration that person will undergo as a function of the stretch distance implemented in the harness.
There's a fall protection video on YouTube that talks a bit about this. It shows how without shock absorption, a person 220 lb person falling 6' will experience 4,000+ lbs of arresting force, but that by using a shock absorbing lanyard, that number is reduced to 832 lbs. I get a little confused here because reducing the arresting force doesn't mean you're allowed to reduce the strength of your anchor point; it still has to be rated for 5,000 lbs. I've assumed that means the anchor point doesn't undergo the same shock absorption as the person falling? I was a liberal arts guy, so the is a gray area for me.
Now, a person dropping off the platform will be falling more or less downwards, so most of the force exerted on the lift platform by the fall arrestor gear will not be in the direction of the 'maximum tippability' force but perpendicular to it.
Is the takeaway, then, that a scissor lift that could tip at 150+ lbs of horizontal force would not likely receive enough horizontal force to tip over if a person tied off inside were to fall over the rail?
posted by jwhite1979 at 1:15 PM on June 8, 2020
Tie yourself up to the ceiling, start kicking off heavy things tied down and see what it takes to topple the lift. This isn't as snarky as it sounds. It's just that it depends on so many things that others have mentioned and you seem to know. How high is it, how heavy is the bottom, how wide is the platform, falling from front or back, how elastic is the restraint, how long is the restraint, what's the initial trajectory of the falling body. There's a whole envelope of possible situations. But the attachment is probably rated so high because engineers in everyday things tend to multiply the minimum by 10x or so as a safety margin. Whereas say rocket scientists launching satellites multiply the minimum by 1.1x margin of safety. The lift might tip, but that attachment point isn't going to fail.
posted by zengargoyle at 2:28 PM on June 8, 2020 [2 favorites]
posted by zengargoyle at 2:28 PM on June 8, 2020 [2 favorites]
The arresting force is permitted to be 900 lbs. For a fall arrest system this is controlled by proper selection of shock absorber combined with knowledge of the users mass. The 5000lb rating isn't there because they expect 5000lbs but rather so that there is a 5X safety factor on the 900lbs (with a little windage for a nice round number).
Unless people are launching themselves off the side of the lift essentially all the force of a fall arrest is going to be straight down as people generally just barely fall off the edge/over the rails.
I want to explain to a supervisor that a lanyard attached to a scissor lift's tie-off point isn't an acceptable fall protection anchor point. I've watched enough safety videos to know this is the case, but I was hoping to be able to prove the concept with some math.
If you are meaning for people in the lift then a shit ton of engineering has been done to ensure the anchor points are adequate. I've never heard of an anchor point on an AWP failing. With a 900lb load and a 5000lb engineered rating a failed point would have to be severely compromised to fail under use.
There are also sorts of caveats when using a fall restraint system on a scissor lift, especially below 30-30 feet, but anchor point effectiveness isn't one of them.
posted by Mitheral at 2:32 PM on June 8, 2020 [1 favorite]
Unless people are launching themselves off the side of the lift essentially all the force of a fall arrest is going to be straight down as people generally just barely fall off the edge/over the rails.
I want to explain to a supervisor that a lanyard attached to a scissor lift's tie-off point isn't an acceptable fall protection anchor point. I've watched enough safety videos to know this is the case, but I was hoping to be able to prove the concept with some math.
If you are meaning for people in the lift then a shit ton of engineering has been done to ensure the anchor points are adequate. I've never heard of an anchor point on an AWP failing. With a 900lb load and a 5000lb engineered rating a failed point would have to be severely compromised to fail under use.
There are also sorts of caveats when using a fall restraint system on a scissor lift, especially below 30-30 feet, but anchor point effectiveness isn't one of them.
posted by Mitheral at 2:32 PM on June 8, 2020 [1 favorite]
Oh also even if you got someone to launch themselves off the lift maximum force on the harness/anchor point won't occur until the lanyard is point essentially pointed straight down. Before that the swing of the mass on the end of the lanyard limits force. Of course slamming into the side of the lift like a tether ball is going to create it's own injuries but it won't cause the lift to fall over.
I've also never heard of a lift tipping over in a fall arrest situation unless there was a compounding factor that likely would have led to the lift falling anyways.
posted by Mitheral at 2:41 PM on June 8, 2020
I've also never heard of a lift tipping over in a fall arrest situation unless there was a compounding factor that likely would have led to the lift falling anyways.
posted by Mitheral at 2:41 PM on June 8, 2020
but that by using a shock absorbing lanyard, that number is reduced to 832 lbs. I get a little confused here because reducing the arresting force doesn't mean you're allowed to reduce the strength of your anchor point; it still has to be rated for 5,000 lbs. I've assumed that means the anchor point doesn't undergo the same shock absorption as the person falling?
At any moment the force exerted on the lanyard (by the person attached to the end of it, and decelerating) is acting on the attachment point as well. Think of it as a tug-of-war: you pull, and the person at the other end feels that force. If they're not moving they're pulling just as hard as you are. Reducing the deceleration reduces the force at both ends, but as you can not be sure that the fall protection worn is adequate and of the shock-absorbing type, hasn't been used before (so the stretch is gone), or for several other related reasons you (as the lift designer) will generally keep a considerable safety margin anyway; increased weight and material use for a few such attachment points is inconsequential when you relate it to the entire lift anyway.
Is the takeaway, then, that a scissor lift that could tip at 150+ lbs of horizontal force would not likely receive enough horizontal force to tip over if a person tied off inside were to fall over the rail?
Indeed. It's hard for me to see how someone would manage to fall off the platform in such a way that they'd exceed those 150lbf horizontally, at least with a shock-absorbing fall arrestor. Note that from the moment the person is entirely free of the platform you would have to consider how far they could have moved horizontally and vertically before the fall arrestor reaches the end of its free length and starts stretching, because that would determine the direction the lanyard is pointing (and going to pull) at that moment. There's a bit of dependence on whether they fall face forward (lanyard for such harnesses should attach at the back) or somehow flip over the railing backwards, but in both cases the total distance will be 5, maybe 6 foot. Then once there's pull on the lanyard it's going to halt (comparatively gradually) both horizontal and vertical movement, resulting in some elongated half-circle and ending straight down from the edge of the platform.
posted by Stoneshop at 2:52 PM on June 8, 2020 [1 favorite]
At any moment the force exerted on the lanyard (by the person attached to the end of it, and decelerating) is acting on the attachment point as well. Think of it as a tug-of-war: you pull, and the person at the other end feels that force. If they're not moving they're pulling just as hard as you are. Reducing the deceleration reduces the force at both ends, but as you can not be sure that the fall protection worn is adequate and of the shock-absorbing type, hasn't been used before (so the stretch is gone), or for several other related reasons you (as the lift designer) will generally keep a considerable safety margin anyway; increased weight and material use for a few such attachment points is inconsequential when you relate it to the entire lift anyway.
Is the takeaway, then, that a scissor lift that could tip at 150+ lbs of horizontal force would not likely receive enough horizontal force to tip over if a person tied off inside were to fall over the rail?
Indeed. It's hard for me to see how someone would manage to fall off the platform in such a way that they'd exceed those 150lbf horizontally, at least with a shock-absorbing fall arrestor. Note that from the moment the person is entirely free of the platform you would have to consider how far they could have moved horizontally and vertically before the fall arrestor reaches the end of its free length and starts stretching, because that would determine the direction the lanyard is pointing (and going to pull) at that moment. There's a bit of dependence on whether they fall face forward (lanyard for such harnesses should attach at the back) or somehow flip over the railing backwards, but in both cases the total distance will be 5, maybe 6 foot. Then once there's pull on the lanyard it's going to halt (comparatively gradually) both horizontal and vertical movement, resulting in some elongated half-circle and ending straight down from the edge of the platform.
posted by Stoneshop at 2:52 PM on June 8, 2020 [1 favorite]
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Seems like a manufacturer's spec to me. I wouldn't estimate this with any expectation of liability protection.
posted by j_curiouser at 10:31 AM on June 8, 2020