Windshield

From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about glass used on vehicles. For the screen on windows, see window screen. For the microphone wind protecting accessory, see Microphone#Microphone windscreens.
Panoramic (wrap-around) windshield on a 1959 Edsel Corsair

The windshield or windscreen of an aircraft, car, bus, motorbike or tram is the front window. Modern windshields are generally made of laminated safety glass, a type of treated glass, which consists of two (typically) curved sheets of glass with a plastic layer laminated between them for safety, and are bonded into the window frame. Motorbike windshields are often made of high-impact acrylic plastic.

Usage[edit]

Split and raked windshield on a 1952 DeSoto. Note the panes of glass are flat.

Windshields protect the vehicle's occupants from wind and flying debris such as dust, insects, and rocks, and providing an aerodynamically formed window towards the front. UV coating may be applied to screen out harmful ultraviolet radiation. However, this is usually unnecessary since most auto windshields are made from laminated safety glass. The majority of UV-B is absorbed by the glass itself, and any remaining UV-B together with most of the UV-A is absorbed by the PVB bonding layer.[1]

On motorbikes their main function is to shield the rider from wind, though not as completely as in a car, whereas on sports and racing motorcycles the main function is reducing drag when the rider assumes the optimal aerodynamic configuration with his or her body in unison with the machine, and does not shield the rider from wind when sitting upright.

Safety[edit]

Automobile windshield displaying "spiderweb" cracking typical of laminated safety glass.

Early windshields were made of ordinary window glass, but that could lead to serious injuries in the event of a crash. A series of lawsuits led up to the development of stronger windshields. The most notable example of this is the Pane vs. Ford case of 1917 that decided against Pane in that he was only injured through reckless driving. They were replaced with windshields made of toughened glass and were fitted in the frame using a rubber or neoprene seal. The hardened glass shattered into many mostly harmless fragments when the windshield broke. These windshields, however, could shatter from a simple stone chip. In 1919, Henry Ford solved the problem of flying debris by using the new French technology of glass laminating. Windshields made using this process were two layers of glass with a cellulose inner layer. This inner layer held the glass together when it fractured. Between 1919 and 1929, Ford ordered the use of laminated glass on all of his vehicles.[2]

Modern, glued-in windshields contribute to the vehicle's rigidity, but the main force for innovation has historically been the need to prevent injury from sharp glass fragments. Almost all nations now require windshields to stay in one piece even if broken, except if pierced by a strong force. Properly installed automobile windshields are also essential to safety; along with the roof of the car, they provide protection to the vehicle's occupants in the case of a roll-over accident[citation needed].

Today’s windshields are a safety device just like seat belts and airbags. The installation of the auto glass is done with an automotive grade urethane designed specifically for automobiles. The adhesive creates a molecular bond between the glass and the vehicle. If the adhesive bond fails at any point on the glass it can reduce the effectiveness of the air bag and substantially compromise the structural integrity of the roof.[citation needed]

Other aspects[edit]

In many places, laws restrict the use of heavily tinted glass in vehicle windshields; generally, laws specify the maximum level of tint permitted. Some vehicles have noticeably more tint in the uppermost part of the windshield to block sunglare.

In aircraft windshields, an electric current is applied through a conducting layer of tin(IV) oxide to generate heat to prevent icing. A similar system for automobile windshields, introduced on Ford vehicles as "Quickclear" in Europe ("InstaClear" in North America) in the 1980s and through the early 1990s, used this conductive metallic coating applied to the inboard side of the outer layer of glass. Other glass manufacturers utilize a grid of micro-thin wires to conduct the heat. These systems are more typically utilized by European auto manufacturers such as Jaguar and Porsche.

Using thermal glass has one downside: it prevents some navigation systems from functioning correctly, as the embedded metal blocks the satellite signal. This can be resolved by using an external antenna. Even mobile-phones can have problems, typically thermal glass will only allow one (1) per mille/‰ (0.0001) of the signal to pass.. compared to a concrete wall with rebars that allows up to 10% of the signal to pass (per mille/‰ 0.0100).

Terminology[edit]

Singled aeroscreen on Bentley Blower No.1

The term windshield is used generally throughout North America. The term windscreen is the usual term in the British Isles and Australasia for all vehicles. In the US windscreen refers to the mesh or foam placed over a microphone to minimize wind noise, while a windshield refers to the front window of a car. In the UK, the terms are reversed, although generally, the foam screen is referred to as a microphone shield, and not a windshield.

Auto windshields less than 20 cm (8 in) in height are sometimes known as aeroscreens since they only deflect the wind. The twin aeroscreen setup (often called Brooklands) was popular among older sports and modern cars in vintage style.

A wiperless windshield is a windshield that uses a mechanism other than wipers to remove snow and rain from the windshield. The concept car Acura TL features a wiperless windshield using a series of jet nozzles in the cowl to blow pressurized air onto the windshield.

Repair of stone-chip and crack damage[edit]

According to the US National Windshield Repair Association many types of stone damage can be successfully repaired. Whether the windshield can be repaired always depends upon four factors: the size, type, depth, and location of the damage.[3]

Size and depth[edit]

Repair of cracks up to 61 cm (24 in) is within permissible limits, auto glass with more severe damage needs to be replaced.

Type[edit]

Circular Bullseyes, linear cracks, crack chips, dings, pits, and star-shaped breaks can be repaired without removing the glass, eliminating the risk of leaking or bonding problems sometimes associated with replacement.

Location[edit]

If the damage is in the driver’s line of sight or very near the windshield’s edge, usually it cannot be repaired.

Some damages are very difficult to repair, or cannot be repaired:

  • on inside of the windshield
  • deep damage on both layers of glass
  • damage over rain sensor or internal radio antenna
  • damages that reach into the driver’s critical viewing area
  • complex multiple cracks
  • very long cracks (i.e. over 18 - 24 inches long)
  • contaminated cracks
  • edge cracks and chips

In cracked windshield repair, air is removed from the damaged area on the windshield with a specified vacuum injection pump. Then using the injection pump, clear adhesive resin is injected to replace the air in the windshield crack. The resin is then cured with an ultraviolet light. When done properly, the damaged area’s strength is restored, as is 90–95% of the clarity. Auto glass chip repair usually take about 15–20 minutes.[4]

Windshield replacement[edit]

Windshields that cannot be repaired have to be replaced by cutting out the old piece of glass and installing a new one. Replacement of a windshield typically takes less than 1 hour. To ensure the vehicle is safe to drive the Safe Drive Away Time has to be respected.The safe drive away time (SDAT) or minimum drive away time (MDAT) refers to the time required until a windshield installation, e.g. after a glass replacement is considered to be safely bonded to drive again.

Airbags deploy at speeds up to 89 m/s (200 mph) and exert tremendous force on the windshields. The front-seat occupants crash into the airbag 50 ms after initial deployment of the airbag.[5] This is the reason, why forces on windshields are mainly due to occupants who impact the airbag. For the safety of occupants, even windshields which have been replaced soon before must be able to withstand the forces of a crash. Thus, the knowledge of the minimum time needed to cure the glass bonding adhesives is extremely important. This safe drive away time (SDAT) or minimum drive away time (MDAT) refers to the time required until a windshield installation, e.g. after a glass replacement is considered to be safe to drive again.[6] Criteria are laid down in U.S. Federal Motor Vehicle Safety Standards 212/208 (see FMVSS) to prove the reliability of adhesive systems. Typically the SDAT is verified with crash tests but also with high speed laboratory test methods.

Recently with the advent of quick drying sealants, mobile windshield replacements have become more prevalent. Many precautions must be taken with mobile installations to prevent driving the vehicle prior to the SDAT or MDAT. The temperature and humidity of the vehicle, windshield and sealants cannot be controlled for mobile installations. Ideally 70 degrees fahrenheit and 50% humidity. Many locations and climates do not naturally permit these temperature factors, the further from the ideal temperature and humidity the longer it can take to seal the windshield and in extreme cases cause the windshield and occupants to be ejected from the vehicle during a crash. This is why no vehicle manufacturer recommends a mobile windshield replacement. A controlled environment ensures the performance and safety of the windshield installation. [7]

Forces of occupants on the windshield are of course lower, if they are belted.[8] As consequence, adhesive suppliers are usually informing their customers about the level of security achieved:

1) Example: Security exceeding FMVSS 212/208 belted

2) Example: Security exceeding FMVSS 212/208 unbelted[9]

Disposal[edit]

Waste disposal of laminated glass is no longer permitted in landfill in most European countries as the End of Life Vehicles Directive (ELV) is implemented. A study[10] by Surrey University and Pilkington Glass proposes that waste laminated glass is placed into a separating device such as a rolling mill where the glass is fragmented and the larger cullet is mechanically detached from the inner film. The application of heat then melts the laminating plastic, usually Polyvinyl Butyral "PVB" enabling both the glass and the interior film to be recycled. The PVB recycling process is a simple procedure of melting and reshaping it. A Yorkshire consortium of CO2Sense and Ecofillers Ltd estimate that in the UK the potential market for recycled PVB in 2020 may reach a value of around £5,000.

See also[edit]

References[edit]

  1. ^ Tuchinda, Chanisada; Sabong Srivannaboon; Henry W Lim (2006). "Photoprotection by window glass, automobile glass, and sunglasses". J Am Acad Dermatol 54: 845–854. doi:10.1016/j.jaad.2006.05.014. Retrieved 23 January 2013. 
  2. ^ "Your Windshield is Not Just a "Wind-Shield" Any More". National Glass Association. Archived from the original on 2 August 2009. 
  3. ^ "Is Your Windshield Repairable?". Car Windshield Info. Retrieved 22 February 2014. 
  4. ^ "Windshield Repair FAQ's". Patsco windshield repair. Retrieved 14 December 2013. 
  5. ^ Bob Stenzel: Determining SDAT, AGRR Magazine, March/April 2012, page 29-30
  6. ^ Cognard, Philippe (2006). Handbook of Adhesives and Sealants. Elsevier. ISBN 978-0080447087. 
  7. ^ http://www.briteglass.com/brite-glass-blog/entry/mobile-windshield-installations-the-ugly-truth
  8. ^ Exposing Industry Shortcuts to SDAT Recommendations, http://www.sika.com/en/solutions_products/Sika%20Industry%20Business/01a001/noshortcutstosafety/nsts_4.html
  9. ^ Premium Adhesive Systems, AGRR Magazine, March/April 2012, Page 15
  10. ^ "Laminated Car Windscreen Recycling" (pdf). Archived from the original on 2008-10-31. 

Manufacturing Processes Reference Guide: Robert H. Todd, Dell K. Allen, and Leo Alting How It's Made: Windshields

External links[edit]