Wednesday, August 19, 2009

Mechanical Power Transmission using Belt Drives and Chain Drives

Major types of flexible mechanical power transmission are belts and chains. Belts operate on pulleys or sheaves, whereas chains operate on toothed wheels called sprockets.

When to use chain drives or belt drives?

Electric motors typically operate at too high speed e.g. 1500 rpm and deliver too low torque e.g. 1.8 N.m to be appropriate for the final drive application. These figures are taken from 0.25 kW motor specs of some manufacturers just to get an idea. For a given power transmission, the torque is increased in proportion to the amount that rotational speed is reduced. So the method of speed reduction is usually required for normal mechanical power transmission system.

Usually, we use belt drives for first stage reduction because of high speed of the motor. A smaller drive pulley is attached to the motor shaft which runs at high speed, while a larger diameter pulley is attached to the parallel shaft that operates at a correspondingly lower speed.

" Usually, we use belt drives for first stage reduction because of high speed of the motor..."
However, we can imagine that for the very large ratios of speed reduction, gear reducers are desirable because they can typically accomplish large reductions in a rather small package. The output shaft of the gear-type speed reducer is generally at low speed and high torque. If both speed and torque are satisfactory for the application, it could be directly coupled to the driven machine.

However, the output of gear reducers must often be reduced more before meeting the requirements of the machine because the gear reducers are available only at discrete reduction ratios. At the low-speed, high-torque condition, chain drives become desirable. The high torque causes high tensile force in the chain. The chains normally made of metal and they can withstand the high forces.

" At the low-speed, high-torque condition, chain drives become desirable..."
In general, belt drives are used where the rotational speeds are relatively high, which results in relatively low tensile forces in the belt. But at lower speed, the tensile force in the belt becomes too large for typical belt cross sections and may lead to slipping between sides of the belt and pulleys (or sheaves)

Tuesday, August 18, 2009

Philosophy of a safe design

Every design approach, we must ensure that the stress level is below the yield in ductile materials, automatically ensuring that the part will not break under a static load.

For brittle materials, we must ensure that the stress levels are well below the ultimate tensile strength.

Two other failure modes that apply to machine members are fatigue and wear. Fatigue is the response of a part subjected to repeated loads. Wear often happens where two parts are in contact with each other such as gears, bearings, and chains, for which it is a major concern.

source: Machine Elements in Mechanical Design, Robert L. Mott

Monday, August 17, 2009

Chain Sprockets

Chain Sprockets are fabricated from a variety of materials; this would depend upon the application of the drive. Large fabricated steel chain sprockets are manufactured with holes to reduce the weight of the chain sprocket on the equipment. Because roller chain drives sometimes have restricted spaces for their installation or mounting, the hubs are made in several different styles.



Type A chain sprockets are flat and have no hub at all. They are usually mounted on flanges or hubs of the device that they are driving. This is accomplished through a series of holes that are either plain or tapered.

Type B chain sprockets has a hub on one side and extend slightly on the other side. The hub is extended to one side to allow the sprocket to be fitted close to the machinery that it is being mounted on. This eliminates a large overhung load on the bearings of the equipment.

Type C chain sprockets are extended on both sides of the plate surface. They are usually used on the driven sprocket where the pitch diameter is larger and where there is more weight to support on the shaft. Remember this the larger the load is, the larger the hub should be.

Type D chain sprockets use an "A" chain sprocket mounted on a solid or split hub. The type A chain sprocket is split and bolted to the hub. This is done for ease of removal and not practicality. It allows the speed ratio to be changed easily by simply unbolting the sprocket and changing it without having the remove bearings or other equipment.

Chain Drives - Conveyor Roller Chain

Chain drives are an important part of a conveyor system. Chain drives are normally used to transmit power between a drive unit and a driven unit of the conveyor system. Chain drives can consist of one or multiple strand chains, depending on the load that the unit must transmit. The chains need to be the matched with the sprocket type, and they must be tight enough to prevent slippage.
" Chain is sized by the pitch or the center-to-center distance between the pins. This is done in 1/8" increments. "
Conveyor Roller Chain
Roller chains are made up of roller chain link that are joined with pin links. The roller chain links are made up of two side bars, two rollers, and two bushings. The roller reduces the friction between the chain and the sprocket, thereby increasing the life of the unit.

Roller chains can operate at faster speeds than plain chains, and properly maintained, they will offer years of reliable service. Some roller chains come with a double pitch, meaning that the pitch is double that of a standard chain, but the width and roller size remains the same. Double-pitch chain can be used on standard sprockets, but double-pitch sprockets are also available.
The main advantage to the double-pitch chain is that it is cheaper than the standard pitch chain. So, they are often used for applications that require slow speeds, as in for lifting pieces of equipment in a hot press application.
Roller chain is ordinarily hooked up using a master link (also known as a connecting link), which typically has one pin held by a C clip rather than friction fit, allowing it to be inserted or removed with simple tools. Half links (also known as offsets) are available and are used to increase the length of the chain by a single roller.

Roller chain is made in several sizes, the most common American National Standards Institute (ANSI0 standards being 40, 50, 60, and 80.
"The first digit(s) indicate the pitch of the chain in eighths of an inch, with the last digit being 0 for standard chain, 1 for lightweight chain, and 5 for bushed chain with no rollers."
Roller chain is used in low- to mid-speed drives at around 600 to 800 feet per minute; however, at higher speeds, around 2,000 to 3,000 feet per minute, V-belts are normally used due to wear and noise issues.

It is advisable either to monitor the exact length of a drive chain (the generally accepted rule of thumb is to replace a roller chain which has elongated 3% on an adjustable drive or 1.5% on a fixed-center drive), or just replace it at established intervals of use to minimize wear on the sprockets. Thus, any savings in maintenance costs from skimping on lubrication result in increased costs for monitoring wear and for replacement. This need for frequent maintenance, comprising lubrication, assessing wear, and replacement of the chain and/or the sprockets, represents the major drawback of the utilization of roller chain.

The lengthening of a chain is calculated by the following formula:

% = [M − (S * P)] / (S * P) * 100

M = the length of a number of links measured
S = the number of links measured
P = Pitch

Some contents from wikipedia.org

Sunday, August 16, 2009

Dowel Pins and Locating Pins

Dowel pins are the fasteners used to secure two parts together. They are available in both Metric and English sizes, and carry specifications such as diameter, length, and materials. Most dowel pins are made of stainless steel, plastic, , hardened steel, or ground steel. Plastic dowel pins are made of thermoplastic or thermosetting polymers with high molecular weight. Stainless dowel pins are chemical and corrosion resistant, and have relatively high pressure ratings.

Dowel pins are often used as precise locating devices in machinery. Stainless dowel pins are machined to tight tolerances, as are the corresponding holes, which are typically reamed. A dowel pin may have a larger diameter so that it must be pressed into its hole or a smaller diameter than its hole so that it freely slips in.

When mechanical design engineers design the mechanical components, typically they use dowel holes as reference points to control positioning variations and attain repeatable assembly quality. If no dowel pins are used for alignment e.g., components are mated by bolts only, there can be significant variation, or "play," in component alignment. Typical drilling and milling operations, as well as manufacturing practices for bolt threads, introduce at least 0.2 mm play for bolts up to 10 mm. If dowel pins are used in addition to bolts, the play is reduced to approximately 0.01 mm.

In automatic machinery, dowel pins are used when precise mating alignment is required, such as in differential gear casings, engines, transmissions and indexing mill. Not only high precision will be achieved, but also it can reduce time to exchange the machine parts. Imagine if you have 50 units of product holders that are required to mount into the indexing mill, without dowel pins, you have to take most of the time to adjust the position of the product holders online. With the dowel pins, you can easily set the units off-line and just mount them into the indexing mill. They are widely used for SMED concept (Single Minute Exchange of Die).

Single Minute Exchange of Die (SMED) is one of the many lean production methods for reducing waste in a manufacturing process. It provides a rapid and efficient way of converting a manufacturing process from running the current product to running the next product. This rapid changeover is key to reducing production lot sizes and thereby improving flow

To locate the parts precisely, normally diamond locating pins are used in conjunction with a round locating pin. The round locating pin holds the part in position, and the diamond pin hold the part to keep it from rotating around the round locating pin.

It is poor design practice to use two round pin four-way locators, as the tolerance stack-up from the center of one pin to the other pin will make mounting the part impossible. Two round locating pins should only be used when one is place in a hole and one is place in a slot. If two holes are to be used as locating features, use one round and one diamond pin.

The long axis of the diamond locating pin should be positioned perpendicular to a line drawn between the center of the round locating pin and the center of the diamond locating pin. Any other orientation will allow the part to swing from side to side and produce inaccurate results.