In some cases the pinion, as the foundation of power, drives the rack for locomotion. This would be common in a drill press spindle or a slide out mechanism where the pinion is usually stationary and drives the rack with the loaded system that needs to be moved. In other cases the rack is fixed stationary and the pinion travels the distance of the rack, providing the load. A typical example would be a lathe carriage with the rack set to the underside of the lathe bed, where the pinion drives the lathe saddle. Another example will be a building elevator that may be 30 tales high, with the pinion generating the platform from the bottom to the very best level.
Anyone considering a rack and pinion app will be well advised to purchase both of these from the same source-some companies that generate racks do not produce gears, and several companies that generate gears usually do not produce gear racks.
The customer should seek singular responsibility for smooth, problem-free power transmission. In case of a problem, the customer should not be in a position where the gear source promises his product is correct and the rack supplier is declaring the same. The customer has no wish to turn into a gear and equipment rack expert, aside from be considered a referee to claims of innocence. The customer should become in the position to make one telephone call, say “I have a problem,” and be prepared to get an answer.
Unlike other kinds of linear power travel, a gear rack can be butted end to get rid of to provide a practically limitless amount of travel. This is best accomplished by having the rack provider “mill and match” the rack so that each end of each rack has one-fifty percent of a circular pitch. This is done to an advantage .000″, minus an appropriate dimension, so that the “butted with each other” racks can’t be several circular pitch from rack to rack. A small gap is appropriate. The correct spacing is attained by just putting a short piece of rack over the joint to ensure that several teeth of every rack are involved and clamping the location tightly until the positioned racks can be fastened into place (see figure 1).
A few words about design: While most gear and rack producers are not in the design business, it is usually helpful to have the rack and pinion producer in on the early phase of concept development.
Only the original equipment manufacturer (the customer) can determine the loads and service life, and control installing the rack and pinion. However, our customers often reap the benefits of our 75 years of experience in generating racks and pinions. We can often save considerable amounts of money and time for our customers by seeing the rack and pinion specifications early on.
The most typical lengths of stock racks are six feet and 12 feet. Specials can be made to any practical length, within the limitations of material availability and machine capability. Racks can be stated in diametral pitch, circular pitch, or metric dimensions, and they can be stated in either 14 1/2 degree or 20 degree pressure angle. Special pressure angles could be made with special tooling.
Generally, the wider the pressure angle, the smoother the pinion will roll. It’s not uncommon to go to a 25-degree pressure angle in a case of extremely large loads and for situations where more power is necessary (see figure 2).
Racks and pinions could be beefed up, strength-wise, by simply going to a wider face width than regular. Pinions should be made out of as large several teeth as is possible, and practical. The larger the amount of teeth, the bigger the radius of the pitch collection, and the more the teeth are engaged with the rack, either completely or partially. This outcomes in a smoother engagement and overall performance (see figure 3).
Note: in see number 3, the 30-tooth pinion has 3 teeth in almost complete engagement, and two more in partial engagement. The 13-tooth pinion offers one tooth in full contact and two in partial contact. As a rule, you must never go below 13 or 14 teeth. The small number of teeth results within an undercut in the main of the tooth, which makes for a “bumpy trip.” Sometimes, when space is a problem, a straightforward solution is to put 12 tooth on a 13-tooth diameter. That is only suitable for low-speed applications, however.
Another way to accomplish a “smoother” ride, with an increase of tooth engagement and higher load carrying capacity, is by using helical racks and pinions. The helix angle gives more contact, as one’s teeth of the pinion come into full engagement and then keep engagement with the rack.
As a general rule the strength calculation for the pinion may be the limiting factor. Racks are usually calculated to be 300 to 400 percent stronger for the same pitch and pressure angle if you stick to normal rules of rack encounter and material thickness. However, each situation ought to be calculated onto it own merits. There should be at least 2 times the tooth depth of material below the main of the tooth on any rack-the more the better, and stronger.
Gears and equipment racks, like all gears, should have backlash designed into their planetary gearbox mounting dimension. If indeed they don’t have enough backlash, you will have a lack of smoothness doing his thing, and you will see premature wear. Because of this, gears and gear racks should never be utilized as a measuring device, unless the application is fairly crude. Scales of most types are far superior in calculating than counting revolutions or the teeth on a rack.
Occasionally a person will feel that they have to have a zero-backlash setup. To get this done, some pressure-such as spring loading-is definitely exerted on the pinion. Or, after a check run, the pinion is set to the closest match which allows smooth running instead of setting to the suggested backlash for the provided pitch and pressure angle. If a person is looking for a tighter backlash than normal AGMA recommendations, they could order racks to particular pitch and straightness tolerances.
Straightness in gear racks can be an atypical subject matter in a business like gears, where tight precision is the norm. Many racks are created from cold-drawn materials, that have stresses built into them from the cold-drawing process. A piece of rack will most likely never be as straight as it was before one’s teeth are cut.
The most modern, state of the art rack machine presses down and holds the material with a lot of money of force in order to get the ideal pitch line that’s possible when cutting the teeth. Old-style, conventional machines generally just defeat it as toned as the operator could with a clamp and hammer.
When the teeth are cut, stresses are relieved privately with the teeth, causing the rack to bow up in the centre after it is released from the machine chuck. The rack must be straightened to make it usable. That is done in a variety of ways, depending upon the size of the material, the grade of material, and how big is teeth.
I often utilize the analogy that “A gear rack has the straightness integrity of a noodle,” which is only a slight exaggeration. A gear rack gets the very best straightness, and therefore the smoothest operations, by being mounted toned on a machined surface area and bolted through underneath rather than through the medial side. The bolts will draw the rack as toned as feasible, and as flat as the machined surface area will allow.
This replicates the flatness and flat pitch type of the rack cutting machine. Other mounting methods are leaving too much to possibility, and make it more challenging to assemble and get smooth procedure (see the bottom half of see figure 3).
While we are on the subject of straightness/flatness, again, in most cases, heat treating racks is problematic. This is especially therefore with cold-drawn materials. Heat treat-induced warpage and cracking is usually an undeniable fact of life.
Solutions to higher power requirements could be pre-heat treated materials, vacuum hardening, flame hardening, and using special materials. Moore Gear has many years of experience in coping with high-strength applications.
In these days of escalating steel costs, surcharges, and stretched mill deliveries, it appears incredible that some steel producers are obviously cutting corners on quality and chemistry. Moore Gear is its customers’ finest advocate in needing quality materials, quality size, and on-time delivery. A metal executive recently stated that we’re hard to utilize because we expect the correct quality, quantity, and on-period delivery. We take this as a compliment on our customers’ behalf, because they depend on us for those very things.
A simple fact in the gear industry is that the vast majority of the gear rack machines on store floors are conventional devices that were built in the 1920s, ’30s, and ’40s. At Moore Equipment, all of our racks are created on state of the art CNC machines-the oldest being truly a 1993 model, and the most recent shipped in 2004. There are approximately 12 CNC rack machines available for job work in the United States, and we have five of them. And of the most recent state of the art machines, there are just six worldwide, and Moore Gear gets the only one in the United States. This assures that our customers will receive the highest quality, on-period delivery, and competitive pricing.