Ihab Saad – Scrapers

So on and so forth. So the productivity is equal to the rated

capacity times the operational efficiency divided by the cycle

time. So let's see how we're going to calculate that.

Here we have

different tables that give us different factors that affect

whether it's the job conditions and

that are going to primarily affect the fixed time and so on. How are

you going to load it? Is it? Push, loaded single engine, push, loaded

twin engine, push, pull, or self loading elevator. In our case, we

have an elevator self loading

scraper. And then we also have, if you remember when we discussed the

trucks and the hauling and so on, if you are going to be operating

on a very short stretch of road, then by the time you reach your

maximum speed, you have to start braking to stop at the end of that

stretch. So we're going to have a factor that's going to affect that

maximum speed. Because once you calculate the maximum speed from

the performance curves, you have to multiply it by that factor. And

in this case, the factor for the short speed is going to be point

four, five is going to affect the maximum speed. It's going to be

only 45% of the maximum speed. And if you have a long stretch of

road, like 5000 feet, then the factor in this case, going to be

96% we are already familiar with this table because we have used it

before,

and what we have here on this slide is again, something that we

have seen before, and we already know how to use it, which is a

performance chart. In this case, it's for a scraper. We have the

weight both loaded and empty. We have the effective grade

represented by these parallel lines. And then we have the

different gears. So we are going to use the weight to intersect

with the effective grade. Go horizontally and check which gear

is that going to be intersecting with, and then that's going to

give us the speed and which gear are we going to be operating

within.

Here's another example of very similar one. Again, here's the

loaded weight, and here's the empty weight,

the push loading scrapers, again, as we have seen before, sometimes

the engine of the tractor in front of the board is not enough to move

that scraper, so scraper sometimes require assistance in loading to

fill the rated capacity. Push, pull. Scrapers can work in tandem

to help each other, as we have seen in the video clip that we saw

a few minutes ago, non elevating scrapers need pushers to load

because they cannot lift the soil on their own. Crawler tractors are

usually used since they have better traction than wheeled

tractors,

especially on when you have a high

resistance, then in this case, the crawlers are going to be better.

We have seen with the wheeled tractor how there was some

slipping, and then it overcame that slipping and started moving

forward. The loading method can be either backtrack loading, chain

loading or shuttle loading. We're going to see some pictures

representing each one of these different types of loading.

So when scrapers are push loaded, the material in the bowl gets

compacted due to the pressure of forcing the material into the

bowl. Now imagine the bowl is moving forward, the blade is down,

so it's being filled as the ball is getting filled, the new soil

compresses the old soil inside the bone already. So the density of

the material in the bone is determined by the equation. The

density is equal to 100%

110%

times the bank density divided by 100% plus swell, which means it's

110%

of the loose

density of that sort.

Now here we have this picture representing the the push

phase. You have here the tractor, and then you have another tractor

here in the back. It pushes it forward. And now, once the scraper

is fully loaded, that pusher goes back to push another scraper. So

they work. They are in parallel. Here you push the first one, and

then only the pusher goes back. Now the tractor is going to move

that ball forward, and the pusher here in the back is going to move

back and it's going to push another tractor. So this is called

the backtrack loading backtrack, because only the pusher goes back

and it pushes another so.

Scrape, whereas in the second one, called Chain loading, basically

the second scraper is standing little bit ahead of the first one,

so you push the first one until it's able to move. The ball is

filled. It moves away, and then the pusher just positions itself

in behind the next scraper, and then keeps pushing it and so on.

So it's called, this is called Chain loading, because it's not

backtracking to go back to the same original position.

The third method is called shuttle loading. So basically, again, you

push the first scraper until it moves out of the way, and then you

go back the other scraper is facing the other way around, so

the other way the other scraper, the second scraper, is moving in

the opposite direction. So you go behind the second scraper and

start pushing it. So this is called shuttle loading because it

goes sort of in a circle or in a closed circuit

to determine the number of scrapers and pushers,

to determine the number of scrapers a pusher can load, we

must determine the pusher cycle time. So the cycle time for the

pusher is the time to contact the scraper, to touch it, and then

time to push it while it loads, and then time to boost it out of

the cut once the ball is filled, and then time to maneuver to

contact the next scraper. And that depends on the method of loading,

if it's going to be shuttle or is going to be backtracking and so

on. If no performance data exists, the cycle time for the pusher can

be estimated as

1.4

LS, which is the loading time for the scraper in minutes plus point

two, five minutes, which is the time to make that contact and to

start pushing that scraper. So it's point it's 1.4 times the

loading cycle time for the scraper, plus point two five

minutes or 15 seconds, basically

the boost time, which is time assisting the scraper out of cut

point one minute return time is estimated to be 40% of load time,

because now the pusher goes back empty. It does not have the same

resistance, so it can come back at higher speed. The maneuver time is

point one, five minutes again to maneuver and to position itself

behind the next scraper. Therefore, the number of scrapers

that a loader can push is determined by the equation n. The

number of scrapers is equal to the cycle time of the scraper divided

by the cycle time of the pusher. CTS is the cycle time for the

scraper, and CTP is the cycle time of the pusher. Let's

look at an example again. It's going to make things easier.

A CAD six, 630, 1e single engine. Scraper will be used to excavate

the side, the side of a large fill to level a construction site. The

soil is sandy clay, weighing 2700 pounds per bank cubic yard, with a

swell of 18%

the scraper has a 450 horsepower turbocharged diesel engine.

The haul road is 4000 feet. The distance is 4000 feet, with an

uphill grade of 3% from the cut area to the dump area. The running

resistance is 8585

pounds per ton, and the coefficient of traction is point

four,

A, D 8n crawler will be used to push the scrapers to load them to

heat capacity. The project site has an elevation of 3000 feet. Now

look at the problem, and each word has a certain meaning. Now here it

gives us an elevation of 3000 feet. In the older problems that

we have solved with other pieces of equipment, we had the derating

factor due to the elevation. But notice here that it told us that

this is turbocharged diesel engine. And we have mentioned

before that turbocharged diesel engines are not affected by the

elevation. So this is sort of a trick, if you do understand the

nature of the engine, and it's not going to be affected by any

derating factor, then this number is totally redundant, and we're

not going to be using

it. The scraper has the following characteristics, the rated

capacity, 31 cubic yards, loose cubic yards, of course, the empty

weight, 96,880

pounds, the maximum load, 75,000 pounds, the weight distribution

when empty on the drive axle is 67% rear axle, 33%

and when loaded again, is sort of balanced. The drive axis, 53% and

the rear axle, 47%

what's the estimated production of the scraper in.

Cubic yards, if the operation efficiency is 50 minutes per hour,

and the scraper does not wait in the cut for a pusher and how many

scrapers can a pusher load? So we're going to need to calculate

the cycle time for a pusher and the cycle time for a scraper to

determine how many pushers are we going to need. This problem might

look familiar, because we have solved something similar to that

when we're talking about haulers, when we're talking about trucks

and the different types of resistance that they're going to

face, and so on and so forth. So we're going to follow exactly the

same steps.

First of all, we're going to check if we're going to be weight

controlled or volume controlled. The density of the material the

scraper will carry is determined by the equation density. Now here

we're going to apply this is going to be unique to the scrapers,

because of the compression factor and the compaction factor that's

going to take place. So the density is 110%

the bank density divided by 100% plus the swell factor, which gives

us basically 110%

of the loose cubic density. Loose density, which is 2517 pounds per

cubic yard. Per loose cubic yard, the weight of the load when filled

to heap capacity, the capacity is 31 loose cubic yards. So the total

weight is going to be 31 times 2517

and that gives a total weight of 78,027

pounds, which is beyond the load capacity of that scraper. Because

the load capacity the maximum weight was 75,000 pounds, which

means we are not going to be able to fill it to the heat capacity of

31

loose cubic yards. So what would be the volume that's going to be

applicable? In this case, it's going to be 75,000 pounds divided

by the density, which gives 29.8 loose cubic yards, which can be

translated into bank cubic yards by dividing by the bank capacity,

75,000 pounds divided by the bank capacity, the bank density, which

gives 27.8

bank cubic yards. So that was the first check. So we learned here

that it's going to be weight controlled, not volume controlled.

The next step is to estimate the cycle time of the scraper. With

average job conditions, we get a loading time that's from the

tables that we have seen a couple of slides before. We get a loading

time of point seven minutes, spotting a delay time of point

three minutes, and a dump time of point five minutes. All of these

from the table, which makes the fixed time point seven plus point

three plus point five. That's 1.5 minutes,

the maximum rim rim pull generated by the scraper is going to be the

coefficient of traction times the weight on the moving axles. So

it's going to be when it's empty, the coefficient of traction is

point four times point six seven, which is the weight on the moving

axle, times the total load, the total weight, one empty, which is

96,880

which gives a rim pull of 25,964

pounds. Now, when it's full, we're going to add the load of the soil,

in addition to the weight of the scraper itself, the coefficient of

traction is still point four. The weight distribution has changed

because of the heavier load on the other axle. So it's times point

five, three times 96,008 80 plus, the weight inside the bowl, which

is 75,000 pounds, which gives 36,439

pounds. So that's the generated rim pole.

Now we have, we can convert that into tons, the total weight into

tons, by dividing by 2000 which is 85.9 tons. That's to calculate the

resistance, because the resistance is. The factor here that we use is

in tons. So the resistance is going to be

the force of the resistance going to be the rolling resistance, plus

the grade resistance, which is 80 pounds per ton, the factor times

the total weight, 85.9 tons, plus the grade 3% times 20 pounds per

ton, per percent slope times the weight in tons, which gives a

total of total resistance of 12,456

pounds. Now the required drain pull to overcome the resistance is

less than the generated drain pole. So basically, the scraper

can move forward. The resistance is not going to be enough to stop

the scraper from moving. We have seen here while it was empty,

25,900

almost 26,000 and when it's full, 36.4

and the required the resistance is 12.4 so we are quite safe now to

catch.

The effective grade, because we're gonna need to plug that into the

performance chart. We divide the running resistance by 20 pounds

per ton, that's a constant. So 85 divided by 20, that's 4.25

plus the already existing grade, 3% that gives us a total grade of

7.25%

when it's loading and when it's moving forward on the way back,

we're going to subtract the grade. So we have 85 divided by 20, which

is 4.25

minus three. So we have still an effective grade of 1.25%

using the effective grade on the performance chart, we get a speed

of 10 miles per hour. To get the average speed, we incorporate the

speed factor from the tables. We mentioned that

for starting from standstill for half of the road and coming to a

stop for the other half. So basically, if the road distance is

4000 we divide it by two. So the factor from the table is going to

be point nine. Two. Therefore the average speed when loaded is going

to be point nine, two times 11. That's 10.12 miles per hour. And

the average speed this should be 11, by the way, not 10. The

average speed one empty on the way back is going to be point nine,

two times 33 miles per hour, which gives 30.3 mile, miles per hour.

And this is basically how we obtain these numbers. We plugged

in the weight one loaded with a 7.25

effective grade, and that gave us this line horizontally. We go

here. We're gonna hit the fifth gear, and that's going to give us

a speed notice for by the way, that the first numbers are in

kilometers per hour, and the lower scale is in miles per hour. So

it's going to be about 11 miles per hour, and that's what what we

use in the equation.

Now, when empty, here's the weight one empty, and we had an effective

grade of 1.25

so basically, if we keep just going, it's not going to hit even

the 1.25 so we're going to use that speed, which is

basically we're going to keep going down until we reach the

maximum speed, which is going to be about 33 miles per hour.

So in the 4000 feet, which we're going to divide by two, as we had

just discussed,

the time is going to be 4000 divided by the 88 to convert the

feet into miles per hour

divided by the speed. And that gives us a time of 4.5 minutes,

moving forward, moving backward, the speed is 30.3 so we have 1.5

minutes. Therefore the variable time is the sum of these two, 4.5

and 1.5 that's six minutes. We had already calculated the fixed time

to be a minute and a half. So the total cycle time is fixed plus

variable. That's a total of 7.5 minutes. The Productivity is going

to be the load per cycle

times the number of cycles.

So

the load per cycle is 27.8 bank cubic yards

times operation efficiency, which is 50 minutes per hour

divided by the cycle time, which gives

the total capacity, or total production, of 185.33

band cubic yards per hour.

The pusher cycle time is equal to 1.4 because we are not given any

information about the pusher. So we're gonna use the equation 1.4

the scraper load time plus point two, five minutes, which is 1.4

times point seven, which is the loading time for the scraper,

plus point two, five minutes. And that gives us and the point seven

minutes is from the table that we have used before. So it gives us a

cycle time of 1.2 minutes. Now if the cycle time for the scraper is

7.5 minutes, and the cycle time for the pusher is 1.2 minutes, so

how many scrapers can one pusher serve? The number of scrapers is

going to be scraper cycle time divided by pusher cycle time. So

that gives 6.25 scrapers, which is going to be rounded down to six

scrapers. So one pusher can serve six scrapers. That's basically how

we calculate the production for a scraper and the number of pushers

and the number of scrapers, and so on and so forth.

I hope that has been

well understood, and we we have seen the solved example. Please

make sure that you try to change the numbers and resolve the

problem, to get more practice and to gain

speed in solving the problems which are going to help you

finishing the problems on the exam.

Well, good luck, and I'll see you in another lecture. Bye.