Update #19 Top of Car Clearances - Counterweighted Elevators (2.4.6)

Bob Desnoyers Elevator & Escalator Inspections, Inc.

CODE UPDATE

Update Warning

Update #19 Top of Car Clearances - Counterweighted Elevators (2.4.6)

(Update #20 Top of Car Clearances - Hydraulic Elevators (3.4.4)...will be found here.)

The code sections for determining electric elevator top of car clearances can be a bit overwhelming at first encounter - but I am going to explain the procedure in a clear and logical way...at least that's the plan! As you may know, one of the key safety features of a traction elevator is that once the counterweight has landed on it's buffer(s) and it's weight is removed from one end of the suspension means the car will stop moving in the up direction - even if the drive fails to turn off. There is insufficient traction to pull the car up into the overhead - the ropes (belts) "break" traction. A person and/or the equipment on top of the car will not be injured or damaged if the correct amount of clearance is provided.

Car Breaks Traction

A Properly Maintained Traction Elevator With Adequate Top Of Car Clearances

This applies to the car landing on it's buffer(s) as well - the counterweight stops moving in the up direction and if provided with enough clearance it will also be free from damage.

There is at least one flaw to this safety feature. If the drive sheave is damaged (the grooves are deformed, severely worn, and/or "rope imprinting" has occurred on the sheave surface) and there is a significant amount of rope on the counterweight side (which equates to a significant amount of weight) enough traction may be available to pull the car into the overhead. I have not observed this phenomenon myself but I have heard from reliable sources that it can occur.

Severely Worn Drive Sheave Corrugated Grooves On Drive Sheave

Cross Section Of A Severely Worn Sheave And
A Corrugated Sheave Produced By "Rope Imprinting"

(Left image "borrowed" from Wirerope Works, Inc. (Bethlehem Wire Rope))

Car Maintains Traction When It Should Not

An Improperly Maintained Traction Elevator With
Adequate Top Of Car Clearances.

This should not become a problem if the equipment is properly maintained.

Now that we have an understanding of this significant safety feature we can begin to analyze the clearance requirements found in Section 2.4. We will be using A17.1-2004 as our reference code. (The 2000 edition and the 2005 supplement to the 2004 edition have the same requirements.)

We will be examining a "common" elevator installation - an overhead traction machine in a standard machine room, 1:1 roping, standard pit and overhead construction - nothing unusual to speak of. We'll keep it simple. I'll mention a few unusual situations as we progress - for now we just need the basics.

The top of car clearance areas that need to be addressed are:

1. Crosshead clearance (2.4.6.2(c))

2. Car top clearance (2.4.6.2(c))

3. Nearest striking point clearance (2.4.6.2(c) and 2.4.11)

4. Refuge space clearance (2.4.12.1)

5. Overhead beams and construction not located over the crosshead clearance (2.4.10)

The following 4 components determine the top of car clearances:

1. Designed maximum bottom counterweight runby (2.4.6.2(a)) - The code does not define "designed maximum bottom counterweight runby". However, we can easily calculate this dimension once we obtain our car top measurements and determine our minimum required clearances. I have never noticed this dimension on a layout drawing (I must admit I have not studied very many electric elevator layout drawings since most of the elevators I have inspected are electrohydraulic) so I suspect it is our job to calculate the maximum bottom counterweight runby. I believe we must first find our minimum clearances and once we subtract them from our measured clearances we can determine the "designed" maximum bottom counterweight runby. It is easy to determine and we will in due time. The counterweight runby data plate mentioned in 2.4.5 will require this dimension.

Counterweight Runby Data Plate

Counterweight Runby Data Plate (See 2.4.5)

Remember, in some cases there is no bottom counterweight runby because the buffer is compressed a small amount ("not to exceed 25% of their stroke" (See 2.22.4.8)) when the car is at the top terminal landing. (See 2.4.2.1)

2. The stroke of the counterweight buffer - This might not be the full stroke if the buffer is slightly compressed when the car is at the top terminal landing. (See 2.4.2.1 - mechanical spring-return type oil buffers only not gas spring-return oil buffers (See 2.22.4.8 as well))

3. One of the following dimensions:

A - If no compensating rope tie-down device is provided - ½ the gravity stopping distance (See 2.4.6.2(d)) or
B - If a compensating rope tie-down device is provide - the distance to which the compensating rope tie-down device limits the jump of the car. (See 2.4.6.2(e))

4. An additional amount according to the area we are providing clearance for:

A - 600 mm (24 in.) for equipment mounted on or in the crosshead (See 2.4.6.2(c))
B - 150 mm (6 in.) for equipment extending more than 600 mm (24 in.) above the car top or crosshead (See 2.4.6.2(c))
C - Guide-shoe assemblies or gate posts for vertically sliding gates must not strike the overhead (See 2.4.6.2(c))
D - 1 100 mm (43 in.) from the car top for the refuge space (See 2.4.12.1)

Let's Try An Example

So let's figure out the minimum clearances for an electric elevator with the following parameters:

Speed = 500 Feet Per Minute
Counterweight Buffer Stroke = 17 Inches
Counterweight Bottom Runby = 12 Inches
Remember - all clearances are measured with the car sill level with the top landing hoistway sill.
Clearance At Crosshead = 72 Inches
Clearance At Refuge Space = 93 Inches
Clearance At Guide-Shoe Assembly = 41 Inches
Clearance At Equipment Installed Above Crosshead = 72 Inches
This elevator does not have any equipment installed above the crosshead except the guide-shoe assemblies, therefore, this clearance is 72 inches as well.
This elevator does not have a compensating rope tie-down device.
This elevator does not have emergency terminal speed-reducing devices.

We will begin by determining the "maximum upward movement" that this elevator, which has a rated speed of 500 feet per minute, can develop. We must combine 3 dimensions to arrive at this figure - the bottom counterweight runby, the stroke of the counterweight buffer, and the "jump" of the car. The "jump" of the car can also be described as ½ the gravity stopping distance. Since we do not have a compensating rope tie-down device we will be using "½ the gravity stopping distance, based on 115% of the rated speed." (See 2.4.6.2(d)(1)) So you see that the speed we will use in the "gravity stopping distance" formula is actually 115% of 500 feet per minute which is 575 feet per minute. I believe this "extra" 15% is added to "fill in" the speed range between the actual speed of the elevator and the governor overspeed switch tripping speed. (See Table 2.18.2.1) If the elevator should overspeed in the up direction and hit the buffer just before reaching the governor overspeed switch setting the extra speed, and therefore the extra amount of "jump", will already be accounted for in the formula. The formula for determining the gravity stopping distance can be found in 8.2.4.

For SI (metric) units it is:

S = 51V²
Where
S = free fall (gravity stopping distance), mm
V = initial velocity, m/s

For Imperial (customary, standard, or English) units it is:

S = V² / 19,320
Where
S = free fall (gravity stopping distance), in.
V = initial velocity, ft/min

Using the Imperial formula we calculate "½ the gravity stopping distance" to be:

(575)² / 19,320
= 330625 / 19,320
= 17.113095 inches (gravity stopping distance)
half of 17.113095 = 8.5565475 or 8.56 inches - This is considered the "jump" of the car.

We will now add 17 inches (the buffer stroke), 12 inches (the counterweight runby), and 8.56 inches (the "jump") for a total of 37.56 inches (maximum upward movement). This is the distance above the top landing that we expect the car to finally rise to following a counterweight buffer engagement. Still with me? Good!

MUM Up ...

As a memory device let's call the "maximum upward movement" of the elevator...

MUM

Which will be equal to the:

Counterweight Runby + Counterweight Buffer Stroke + "Jump"

In our example the MUM will be equal to:

12 inches + 17 inches + 8.56 inches = 37.56 inches.

MUM should be easy for our British friends to remember!

So let's see if our clearances are sufficient for this installation.

Remember all of our clearance measurements are taken with the car sill level with the top landing hoistway sill.

The minimum clearance above the crosshead is our MUM plus 24 inches which totals 61.56 inches. We have a clearance measurement of 72 inches so we are good by 10.44 inches.

Crosshead Clearance

With A Measured Clearance Of 72" We Have Plenty Of Crosshead Clearance

The minimum clearance above the refuge space is our MUM plus 43 inches which totals 80.56 inches. We have a clearance measurement of 93 inches so we are good by 12.44 inches.

Refuge Space Clearance

With A Measured Clearance Of 93" We Have Plenty Of Refuge Space Clearance

Let's check our guide-shoe assembly clearance, which happens to be our "nearest striking point" clearance as well. We have a clearance measurement of 41 inches. We need just enough to keep the guide-shoes from striking the obstruction. How about adding 0.06" (1 mm) to our MUM? The total is 37.56 inches plus 0.06 inches - 37.62 inches. We have a clearance measurement of 41 inches so we are good by 3.38 inches.

Guide-Shoe Assembly Clearance

With A Measured Clearance Of 41" We Have Plenty of Guide-Shoe Assembly Clearance

Guide-Shoe Assembly Heads Up ...

Part of 2.4.6.2(c) reads, "...but in no case shall there be less than 150 mm (6 in.) clearance above the equipment, exclusive of guide-shoe assemblies or gate posts for vertically sliding gates,..."

I interpret "exclusive of guide-shoe assemblies or gate posts for vertically sliding gates" as

Exclusive of:
1. Car guide-shoe assemblies
2. Gate posts for vertically sliding gates

Since the code does not provide a definition for guide-shoe assemblies, I will use the National Elevator Industry Educational Program's "Elevator Terms An Illustrated Glossary" for definitions to bolster my point.

Guide Shoes: devices used mainly to guide the car and counterweight along the path of the guide rails

Guide Rollers: guide shoes which use rollers that rotate on the guide rails rather than slide on the rails

If it were up to me I would rewrite the passage as:

"exclusive of car guide-shoe assemblies or gate posts for vertically sliding gates"

As far as I'm concerned any other interpretation of that passage does not make sense. Now I realize that some other inspectors do not agree with me...so check with the authority having jurisdiction in your area for their interpretation.

Roller Guide and Gate Post Clearance

As I Interpret It

Designed Maximum Bottom Counterweight Runby Heads Up ...

I promised to show you how to determine the "designed maximum bottom counterweight runby." Here's how it's done. Once you measure your actual clearances and determine your minimum clearances you find the area with the smallest difference between the actual and the minimum - add that difference to the current counterweight runby and you have the "designed" maximum. (Remember the maximum counterweight runby can not be greater than 900 mm (35 in.)) (See 2.4.4(b)) For instance, our "tightest" difference was 3.38 inches for the guide-shoe assembly. Add the current runby - 12 inches - to 3.38 inches and you get 15.38 inches (391 mm). This is the designed maximum bottom counterweight runby.

Completed Counterweight Runby Data Plate

Properly Marked Counterweight Runby Data Plate (See 2.4.5)

Overhead Beams Not Over Crosshead Heads Up ...

So what do you do if "overhead beams or other overhead hoistway construction, except sheaves, are located vertically over the car, but not over the crosshead?"

Part 2.4.10.1 reads, "The clearance from the car top to such beams or construction, when the car is level with the top landing, shall be not less than the amount specified in 2.4.6 and 2.4.7." We have been dealing with 2.4.6 - counterweighted elevators. 2.4.7 deals with uncounterweighted elevators.

Part 2.4.10.2 reads, "Such beams or construction shall be located not less than 600 mm (24 in.) horizontally from the crosshead." I interpret this to mean:

If the beam or construction is within 24 inches horizontally from the crosshead then the minimum crosshead clearance measurement must be taken from the crosshead to a plane even with the bottom of the beam or construction.

Overhead Beam Less Than 24 Inches From Crosshead

Overhead Beam Less Than 24 Inches From Crosshead
(See 2.4.10)

If the beam or construction is 24 inches or more horizontally from the crosshead then the minimum crosshead clearance measurement must be taken from the crosshead to the bottom of the next obstruction that is within 24 inches horizontally of the crosshead.

Overhead Beam 24 Inches or More From Crosshead

Overhead Beam 24 Inches or More From Crosshead
(See 2.4.10)

It seems to me that the code is written to prevent a "shear zone" between the crosshead and any obstruction above it.

Projections Above The Crosshead Clearance Heads Up ...

So what do you do if you have equipment projecting above the crosshead or car top?

Part 2.4.6.2(c) reads, "600 mm (24 in.) or the distance that any sheave or any other equipment mounted in or on the car crosshead projects above the top of the car crosshead, whichever is greater, but in no case shall there be less than 150 mm (6 in.) clearance above the equipment, exclusive of guide-shoe assemblies or gate posts for vertically sliding gates, mounted on the car top or in or on the car crosshead when the car has reached its maximum upward movement.

So if a 2:1 sheave is mounted on the crosshead the following clearance would need to be provided above the sheave or sheave guard if installed. This clearance requirement also applies to any other device or equipment mounted above the crosshead or car top.

If we have a clearance above the 2:1 sheave of 49 inches and we subtract the minimum required clearance of 43.56 - the MUM plus 6 inches - we are good by 5.44 inches.

Projection Above The Crosshead Clearance

With A Measured Clearance Of 49" We Have Plenty Of Equipment Projecting Above The Crosshead Clearance

Compensating Rope Tie-Down Device Heads Up ...

So what do you do if you have a "compensating rope tie-down device?"

2.4.6.2(e) requires you to find, "the distance to which the compensating rope tie-down device, if provided, limits the jump of the car when the counterweight strikes the buffers at speeds specified in 2.4.6.2(d)."

The speeds mentioned are once again 115% of the rated speed. The tricky part of this requirement is, "What is this distance and how do you go about finding it?"

The image below depicts an example of a compensating rope tie-down device. As you can see it is basically a pawl and ratchet attached to a sheave in the pit. One end of a set of ropes is attached to the bottom of the car and the other end to the bottom of the counterweight. If the ends of these ropes try to move away from each other, for instance during a safety test or buffer test, the pawl keeps the sheave in the pit and limits its upward movement. Another way to accomplish the "tie-down" requirement is to replace the pawl and ratchet with an instantaneous safety device. The knurled-rollers would be designed to grip the compensating sheave guide rails as the sheave is pulled up. I have seen both types of devices and I am sure there are other ways to accomplish the "tie-down."

Since the pawl and ratchet device will limit the movement of the sheave from 0 to approximately 3 inches, which is equivalent to 0 to 6 inches of rope movement, and we must also consider the rope stretch produced by the strain of the car and counterweight pulling on either end of the compensating ropes, you can see that it is difficult to come up with an accurate dimension to add to our MUM. (See #2 and #3 in the example below)

I would recommend that you have the manufacturer of the elevator verify this dimension in writing. They should have tested the "tie-down" device and are probably providing the information to other authorities having jurisdiction and installation mechanics. Let them provide it for you. The worst case scenerio is the maximum distance between ratchet teeth times 2 (6 inches in our example) plus the rope stretch from the upward pull of the car on counterweight buffer engagement.

Compensating Rope Tie-Down Device

Example of a Compensating Rope Tie-Down Device

Incidentally, this compensating rope tie-down device is required to have a "switch or switches mechanically opened by the compensating-rope sheave before the sheave reaches its upper or lower limit of travel." (See 2.26.2.3)

Last But Not Least Heads Up

2.4.11 reads, "When the car crosshead, or car top where no crosshead is provided, is at a distance equal to that specified in 2.4.6.2(c) from the nearest obstruction above it, no equipment on top of the car shall strike any part of the overhead structure or the equipment located in the hoistway."

Put Your MUM On A Stick! ...

Once you know the dimension of the MUM you could cut a stick of EMT or an actual stick to that exact length and use it as a tool. For example, the MUM stick must only clear the guide-shoe assembly and/or the gate post.

MUM On A Stick!