IMAGE SENSORS IN A DIGITAL CAMERA

The image sensors in a digital camera

The image sensor in a digital camera replaces the film.There are two main types of sensors used in D-SLR camjeras. They are the CMOS (Complementary Metal Oxide  Semiconductor) and the CCD (Charge Coupled Device). Both sensors have their particular idiosyncrasies, and they have various characteristics that should be taken into account when purchasing. So it’s important to look at some example files and research the characteristics of the system you intend to purchase. When professional photographers are choosing and purchasing a camera system, they like to shoot some  comparison test shots with the cameras they’re considering. Where possible, emulate this practice. Check the  files in the image-editing software on your computer.Make sure both cameras are tested with all in-camera sharpening turned off to allow for a fair comparison. A camera technician at your local store can show you how to do this. Some sensor/camera combinations are particularly good for low light when using a sensor sensitivity of 400 ISO or higher, while others are fantastic in full natural light and terrible when used with a high ISO sensor sensitivity. Check the amount of “noise” or“grain” at a higher ISO. Do your research well and choose a suitable sensor for the type of photography you’re most interested in. Whereas in the era of film you would have bought your camera and decided on the type of film required at ? a later stage, now you have to make this important decision at the outset. It’s not just about the file size your intended camera is capable of. For instance, if the sensor is less sharp than the alternative camera, or the color characteristics less favorable, you could be unhappy with your choice. Some D-SLRs have in-camera sharpening to compensate for anti-aliasing filters, the main source of “unsharpness.” Initially you wouldn’t think that the physical size of the image sensor would be a factor to consider, since the quality of the file would seem to be the governing factor. However, the smallest sensors on a D-SLR are 18mm x 13.5mm, compared to the format of a traditional 35mm film camera, which is 24mm x 36mm. In this case, a 50mm lens, which on a film camera would constitute a standard lens, becomes a 100mm short telephoto. This initially may seem to be an advantage, since you won’t need long telephotos. But there is an issue with wide angles. A 15mm is an extreme wide angle with a  35mm film camera. With a small sensor, this is only a  slightly wide angle, equivalent to a 30mm lens on a 35mm film camera in some digital cameras. Some manufacturers are addressing this and are beginning to produce special lenses specifically for digital bodies, such as the extraordinary Olympus Zuiko Digital ED 7-14mm f/4.0. ?

BEARING REMOVAL AND CLEANING

Proper bearing removal and cleaning can mean the difference between a performance and bearing failure. If done incorrectly, either procedure can damage the bearing, shaft or housing before installation even begins.

Bearing removal

There are a number of recommended tools and methods that assure safe, reliable bearing removal. Using hammers and drift pins, or an uneven application of force, for example, can cause as much damage to the shaft and housing as to the bearing itself.

Safe bearing removal tools include:

1. Arbor press;

2. Mechanical (or hydraulic) jaw-type pullers that grip parts by their press-fit edges;

3. Mechanical (or hydraulic) push-pullers that use forcing screws to push or pull parts out of the housing

4. Slide hammer pullers with a weighted sliding handle. It strikes a “stop” on its own rod and removes parts from blind holes

5. Special purpose pullers for timing gears, crankshaft sprockets, flywheel pilot bearings.

Selecting the right puller for bearing removal depends on how well the part can be gripped, how much reach and spread (height and width) are needed, and how much power or force is required. To aid in bearing removal, accessories such as extensions for jaw and cross-block pullers, attachments that split bearings, and shaft protectors are also required.

Here are some suggested methods for safe, reliable bearing removal.

Arbor press method

An arbor press applies great force, so it requires little manpower to remove bearings. It is a good method to use where one or both races have been press-fit during installation

An arbor press can be set up to:

• Support the bearing while the press forces the shaft out of the bearing, or;

• Support the shaft while the bearing is being forced off the shaft.

To remove a bearing with a press-fit inner race, first support the inner race on the press base plate with a bar or ring. Using the press, apply force only to that race. This should loosen the race and force the shaft out of the bearing. With two separable races, such as

tapered roller bearings, both races may have been press-fit and should be loosened. Never apply force to the slip fit race or the cage.

Bearing pullers

Bearing pullers should be used when the shaft is too large, obstructed or inappropriate for removal with an arbor press. Mount the puller so that the grip is firm and the puller is square with the surface. Again, apply force only through the press-fit race. The bearing should start to give and be easy to pull out of the housing. With either the bearing pullers or the arbor press, be sure to cover the parts around the bearing to prevent them from damage. Be sure to wear safety goggles to protect yourself against eye injury.

Methods not recommended

Hammering the inner race and flame heating the bearings are two commonly used, but dangerous, methods of bearing removal. The practice of hammering or prying to force removal can be costly and dangerous. The blunt force of the hammer can damage the shaft as well as the bearing. Even when a wood block is used to muffle the blow, splinters and wood chips can get into the housing and damage the bearing.

Using a torch to remove parts is even more dangerous. In addition to the possibility of igniting grease, oil or gas, the intense heat and open flame can weaken component parts and cause subsequent failures.

Cleaning bearings

Soak the bearings in a metal basket suspended in a clean container or tank holding a recommended solvent, overnight if possible. If a basket is not available, suspend the bearings with a wire or place them on a metal plate at the bottom of the container. Do not rest the bearings directly on the bottom of the bucket. (They may not clean as efficiently due to sediment on the bottom of the container.) Under ordinary conditions you can use recommended solvents for cleaning bearings. Oils heavier than SAE 10 should not be used. Gasoline or high flash point naphthas should never be used; they are flammable as well as carcinogenic (that is, they have been known to cause cancer if handled or inhaled).After dirt and grease are removed, rinse the bearings in another clean bucket of solvent. The bearings should then be thoroughly dried. The safest method is natural air-drying. Compressed air, which is free from condensed moisture, may be used to blow out the bearings, but only after all dirt and chips have been removed .If compressed air is used, do not allow bearings to spin and always wear safety glasses to protect your eyes from injury. Caution: Equipment must conform to OSHA standards.

After cleaning, inspect the bearing thoroughly for nicks, leftover dirt and damage. Inspected bearings, which are considered “good” may be used again. However, if re-assembly can not be done immediately they should be protected. Dip the cleaned bearings in a protective lubricant or coat all surfaces with a light grease. Rotate each bearing to work the grease thoroughly in and around the roller and on the races. Then wrap the bearings in waterproof paper and place each in a clean box or carton. If cartons are not available, just wrap them in waterproof paper. Mark the outside of each package to identify the bearing enclosed. Bearings which have a shield or a seal on only one side should be washed, inspected, and handled in the same manner as bearings without

shields or seals. Bearings with shields or seals on both sides should not be washed. Instead, wipe them off to keep dirt from getting inside. Smooth rotating bearings can

be coated with a protective lubricant, then wrapped and s

BALL BEARING TYPES

As a group, ball bearings have many uses in trucks, cars, and off-the-road vehicles. Some of the most common are in steering assemblies, transmissions and differentials. In other applications, such as heavy-duty wheel hubs, they have been replaced by roller bearings.

In addition to the inner and outer races, rolling element and separator, there are three accessory components frequently used with ball bearings:

• Snap rings

• Shields

• Seals

Snap rings are separate components used to locate ball bearings in the housing. They hold the bearing in place the same way a shoulder on the shaft or in the housing would.

Shields are circular rims that cover the open space between the two races, on one or both sides of the bearingThey are attached to the edge of only one race, with clearance left at the inner race. Shields prevent dirt and particles from getting in the bearing, while letting excess lubrication flow through the bearing and escape if necessary.

A seal is a metal-based ring lined with a single, double or triple lip made of rubber, elastomers, synthetic or non-synthetic materials. It is a barrier designed to retain lubricants while excluding moisture, fine dirt, dust, or other contaminants from damaging the bearing. Unlike shields, seals prevent lubricants from leaking out of the bearing.

Operation

Though ball bearings and roller bearings share the same objective – to lessen friction – their strategies are quite different. The mechanical forces underlying ball bearing operation are simple to understand. When a ball bearing is inactive and still, the load applied will be distributed evenly through the races and balls on the contact area. Once the bearing is nudged by a moving load, the ball starts to roll. Material in the race bulges out in front of the ball, then flattens out behind the ball. The ball flattens out in the lower front quadrant, then bulges in the lower rear quadrant. This process continues for each ball as long as the load is in motion.

Continual metal-to-metal contact between the balls and races will eventually wear down the parts and result in bearing failure. So even in doing its job – to lessen friction between two surfaces – the bearing creates its own internal friction. This is one reason why lubrication within the bearing is critical in relieving friction. There are four different ball bearing types used in automotive and fleet applications:

• Single row

• Angular contact

• Double row

• Ball thrust

Single row ball bearing

The single row is one of the most popular ball bearing designs. A crescent-shaped cut in both the inner and outer races forms a wide groove in which a single row of balls roll Though designed primarily for radial load capacity, this bearing can support substantial thrust loads in either direction, even at high operating speeds Careful alignment between the shaft and housing is critical to its performance. The bearing is available with seals and shields for extra protection against contaminants, plus retention of lubricant. A variation of the single row bearing is the maximum capacity bearing. Additional balls can be assembled in the bearing for greater radial load

capacity .However, the extra loading area limits the bearing’s thrust load capacity.

Benefits/advantages

 Good performance under radial loads

 Deep groove permits thrust load capacity in either shaft direction

Assures contaminant-free operation when seals are mounted on the bearing

Applications

• Transmission

• Alternator

• Differential

• Steering gear

• Air conditioner clutch

Angular contact ball bearing

The angular contact ball bearing features two high thrust supporting shoulders – one on the inner race, the other at the opposite side on the outer race. The two shoulders form a steep contact angle slanted toward the bearing’s axis, assuring the highest thrust capacity and axial rigidity. This design can support a heavy thrust load in one direction, sometimes combined with a moderate radial load.

Benefits/advantages

High thrust capacity

Axial rigidity

Applications

• Clutch release

Double row ball bearing

The double row ball bearing combines the design principles of the single row and angular contact bearings. Like the angular contact bearing, it has grooves in the outer and inner races which are positioned so that the load lines through the balls form either an outwardly or inwardly converging angle of contact  The two rows of balls assure a lower axial displacement than the single row design. That is, the bearing is less likely to become misaligned on the shaft or in the housing. The double rows can support heavy radial loads and work well under thrust loads in either direction.

Benefits/advantages

 Thrust capacity in either direction

 High radial capacity

 Less axial displacement

Applications

• Air conditioner clutch

Ball thrust

Designed primarily for clutch release applications, the ball thrust bearing has high thrust capacity. The load line runs parallel through its balls to the shaft axis, so there is little axial displacement Flat shoulders on the shaft and housing are recommended under heavy loads.

Benefits/advantages

 High thrust capacity

 Minimal axial displacement

Applications

• Clutch release

BEARING INSTALLATION

 Bearing Installation

Cleanliness, proper tools, and specific mounting guidelines are needed to assure proper installation as well as long-lasting bearing performance. Improper bearing installation is a common cause of premature bearing failure.

Pre-installation

Check the shaft and housing

A bearing cannot operate properly if the shaft or housing is not in good condition. Before mounting the bearing, be sure shaft and housing bore dimensions are within recommended tolerances. The bearing seat in the housing bore should be perfectly round and not tapered. The shaft and housing also should be clean and free from nicks and burrs .Extra care should be taken when mounting a bearing in a solid housing. Before any installation pressure is applied, the outer race should be perfectly square with the housing bore.

Check the seal

Check the seal, which will be mounted on the shaft. Also check the shaft. Its condition is just as crucial to correct seal placement as it is to bearing operation. Be sure to follow proper seal installation guidelines and use only the recommended tools. Always replace used seals with new ones. Use the same seal design and size as the original. A seal installed next to the bearing is a sure way to prevent fine dirt, dust, moisture and contaminants from reaching the bearing, while also retaining lubricant.

Bearing assembly

With all parts ready for assembly – bearing, shaft and housing – installation can now begin. Do not remove the bearing from its container until you’re ready to install it. Everything must be clean – tools, hands, work area, shaft and housing

2). Then take the bearing out of its protective wrapper, place it on clean paper and cover it with a lint free cloth or oiled paper. The protective grease or oil coating on the bearing should not be removed. This protective coating was put on by the manufacturer to prevent corrosion, dirt or dust from damaging the bearing before and during use and is compatible with all lubricants. Coat the bearing, housing and shaft with the same lubricant being used in the machinery in which it will be placed. This will ease mounting and prevent rust from building up at the press-fit contact area.

Press-fitting the bearing

Press-fit refers to the amount of interference between the race and the seat: the inner race with the shaft, and the outer race with the housing. If the inner race is press-fit then it will rotate with the shaft. If the outer race is to be press-fit then it will rotate with the housing. One example is in a truck’s front wheel hub. Here, the inner race is mounted with a slip fit on the shaft, while the outer race is press-fit  A press-fit is accomplished by stretching the inner race over a shaft slightly larger than the bore of the bearing. Press-fits that are too tight can also be damaging to the bearing. Too tight a fit squeezes the two races together, preventing the balls or rollers from turning correctly and causing excess heat and wear. The result is premature bearing failure. In applications where only one race is press-fit, the other race gets a slip-fit – or a slightly looser fit. This slip-fit is just as important as the press-fit. When the race is too loose, it will creep up on the shaft or in the housing causing it to slam into the surface on which it is stationed. This results in friction, overheating, excessive wear and contact erosion between the shaft and inner race, or housing and outer race.

Mounting the bearing

Start the bearing on the shaft with the rounded corner of the bearing going on first. Fit a clean pipe over the shaft so that it rests only on the race being press-fit. Be sure the bearing is square on the shaft. Then apply pressure to the press-fit race only. Push the press-fit race firmly against the shoulder on the shaft. If the cup is to be press-fit, for example, apply pressure only to that race as you drive it into the housing.

Arbor press

The arbor press is one of the best means of mounting bearings and races .Its action is rapid and pressure can be applied continuously. During bearing installation be sure to support the inner race with two flat bars placed between the inner race and the press’s adaptor plate. Special precautions should be taken when using the arbor press to align the race squarely on the shaft. Too much pressure exerted by the press could easily cause the race to crack or the shaft to become severely scored. Accessory equipment such as drive plates, tubing or pipes, which will carry the force through the press-fit race, should be used whenever possible

Drivers

Drivers may be used for assembling cups, cones and tapered roller bearings. Drivers assure easier assembly by straightening the cups or cones. They also will prevent damage to bearing cages and internal parts  To use the driver method, first separate the cup from the cone. Apply pressure to the races only, and drive each into position. Be careful not to hit the cage. Pressure against the cage will distort and loosen it, causing slanting of the rollers and premature bearing failure. Never use hammers or drift pins directly on the surface of the bearing. If a hammer must be used to mount the bearing, apply pressure to a drive block, adapter sleeve, pipe or tube placed above the bearing. Direct blows to the bearing can cause cocking, denting, cracking and bearing failure

Clutch release bearing: a special mounting procedure

The procedure for mounting a clutch release bearing onto a carrier or sleeve that will be installed in a vehicle is somewhat different from other installation methods. First, lubricate the bearing shoulder on the carrier with a few drops of oil. Place the bearing on a clean drill press table, with the clutch finger face down. Be sure all parts are square. Start the bearing carrier or sleeve into the bore of the bearing by hand. Shift the drill press into the lowest available spindle speed, and close the chuck completely without the drill bit installed. While the spindle is turning, feed the chuck into the bore of the bearing carrier or sleeve until the bearing is completely seated If the chuck diameter is too small or too large, use a small shanked pilot clamped in the chuck to seat

the bearing. Finally, pack the carrier or sleeve with grease before installing

it in the vehicle.

Installation checklist

1. Work only with clean tools, clean hands and clean surroundings to avoid damage to the bearing.

2. Shaft seat and housing bore should be clean, smooth, with the correct dimensions.

3. Leave bearings in the package until ready for assembly. Do not wash off the lubricant covering them.

4. Lubricate the race being press-fit, and shaft or housing seat on which it will sit.

5. Start the bearing on the shaft with the rounded corner of the race going on first.

6. Apply even, driving pressure directly only to the race being press-fit. Be sure that pressure is straight and square.

7. Never hammer directly on races or rollers. Do not use a wooden or soft metal mallet, as chips or splinters may enter the bearing.

8. Use smart, quick taps rather than heavy ones.

9. Be sure all driving accessories and fixtures have straight, square ends.

10. Drive races solidly up against the shoulder of the shaft and housing.

11. Pre-lube bearing prior to installation