Rear window cracks on dump truck 6

[tmusliah]

I am a production engineer working at an earth moving equipment manufacturer, primarily dump trucks. I am primarily involved in the manufacturing of the operator cabins (cabs).

Until recently we used to manufacture the cab entirely out of steel, but our latest model uses a GRP shell over a steel frame.
The windows are bonded, much the same way other automotive companies do it. We use Sika to bond the windows to the fibre glass.

The problem we have been having recently, is the fact that the rear windows have started to crack. Initially this was put down to stone impact, but this theory was shortly dispelled.

These cracks do not occur on all machines, but it seems as if there is some sort of wierd occurence that the machine goes through that causes some sort of flexing in the glass.

The glass is question is a flat sheet of laminated glass 4mmx4mm with vinyl inbetween.
The dimensions of the glass are 780mm x 1260mm, and it is only supported on its extremeties.

As a test we put some glass out in the field wth a larger cross curvature. It was originally 5-10mm, and it is now between 10-15mm. This seems to have increased the lifespan of the glass.

Is there anyone out there, who might suggest a possible cause and solution for this problem.

 

[rmw]

Here is my guess. When your cabs were steel, they were rigid enough so that the glass was not stressed, and hence did not crack. Now, with the GRP cabs, they are more flexible, and as vehicle loads, torsions, etc., are transmitted into the cab, it flexes, and the glass doesn't, so it cracks. The curvature of the new glass has more rigidity, but evidently not enough for the flexure of the new cab material. It is just a guess.

 

[Metalguy]

I think the curvature of the glass allows it to bend more before it cracks. Can't you use plexiglass or some other clear/tinted plastic for the rear windows?

 

[finman]

I'll go for that rmw.

Would it be possible to mount the GRP Cab onto the support structure with resilient mounts, isolating the moulding from the source of applied stresses, once again, just an idea.

 

[tmusliah]

Hi,

Thanks for the response.

The cab is mounted on isolators, but due to the different site conditions, the transmitted frequency cannot be accurately predicted.

I have considered using a Lexan rear window, but our current machine design also has the exhaust stack running up, passed the window. I have been told that due to the heat the Lexan, and other plastic's would soon become opaque.

 

[smckennz]

Perhaps where the cracks start may give you a hint to the cause. If they all start from the same place, then some stiffening may be in order. Or more flexibility in the GRP by drilling a few holes and filling with sika.
Alternatively, increased flexibility in the mounting system may help.Perhaps using a conventional rubber gasket and then using sika to attach to the body.

Seen this on buses and know it can be a big problem.

You need flexibility between glass and body.
I am certain any form of plastic window will wind up giving your company a bad name.

Cheers

Steve

 

[hacksaw]

if you are getting enough heat to opacify lexan, then you are likely creating serious problems for your glass window due to differential expansion in combination with cab flexing.

 

[tmusliah]

Hi Steve,

Yes, majority of the cracks originate from the middle, bottom edge of the glass.
For this reason we have (as a test) put a GRP brace down the center of the cab (vertically) so that we split the glass into 2 smaller pieces, ie. less flexing between the Lhs and Rhs vertical edges.

Would you be able to elaborate on the arrangement of the rubber gasket and sika?

Is there any chance that the shore hardness of the sika (not sure what this value is) makes it less flexible?

Thanks

Trevor

 

[Rich2001]

Use Photoelastic techniques such as shown here

http://www.stressphotonics.com/applications.html

to measure and visualize stresses in the glass.


Purcahse a pair of polarized lenses

http://www.edmundoptics.com/IOD/DisplayProduct.cfm?productid=1396

or

http://scientificsonline.com/product.asp?pn=3038605

and check for stress patterns. Fishing Glasses will somewhat work.

 

[Mandrake22]

We make ag equipment and have been having cracking issues on our steel cabs. We attributed this to stresses induced when operating over rough ground.
We have tried these Lord mounts. So far, they look good.

http://www.lordcorp.com/DesktopDefault.aspx?tabid=534

 

[poltergeist]

Check the racks that the glass comes in. If there is a support in the same area where your cracks are occurring (centre bottom - probably there is something there), the racks could be putting a stress riser in the glass. Make sure they are not tightened down too much against the support, and make sure it is fully padded. Do you also have broken backlights right out of the racks? Number the racks and find the culprits. Get on the supplier and check the polishing of the edges - is the initiation point where anything in the process starts/stops/overlaps (urethane application, for example)?
This is all stuff that is quick to check and easy to fix. Solving torsional stiffness problems as mentioned above may be necessary, but make sure you know what the problem is before spending a lot of time and money.

 

[smckennz]

Hi tmusliah
re the rubber gasket, I was thinking of one like is used on older style vehicles which fits over the edge of the glass right round the perimeter. By fitting one of these and then gluing it to the body, there would be increased flexibility at the edge and less likelihood of cracking (assuming the glass doesnt pop out of the gasket).
It the cracks always start at the same place, then instead of stiffening, perhaps increased flexibility, and especially edge clearance may help. Persumably the window fits in a recess in the FRP body. If the fit is too neat, then a small body deflection can cause the recess to press on the edge of the glass.
Not sure if Sika comes in different hardness grades, but the normal grades are pretty flexible compared to glass and FRP.
The thinner the sika joint, the less flexible.
If you dont like the gasket idea (would look ugly) then how about gluing a strip of foam rubber to the glass and then gluing the other side if the foam rubber to the FRP. This would use the foam rubber for the flex element and would also prevent the glass from being "nipped" in the corner of any recess.
Cheers

Steve

 

[MouseTrap]

About using Lexan (polycarbonate) as a replacement, I would think that the plastic would be very susceptible to scratching - especially with a dump truck - can you say dirt??? - not sure as drivers need to see out that back widow, or whether the dump box actually covers the window when the box is in the down position. Seems all's it would take to incur scratches would be for a driver to wipe away grit/dirt, especially when dry, to put scratches into the 'Lexan' (polycarbonate).

 

[Mandrake22]

As MouseTrap says, lexan will surely scratch.
There is hope however!
One of the auto related design magazines had a blurb this month regarding a "several micron" thick layer of silicon deposited onto lexan. The auto makers like this because it opens them up to more "stylish" glass surfaces. The article pointed out that most of the auto show concept cars have lexan "glass" but that this of course wouldn't be viable for production vehicles due to the scratching issue.