When designing various machines, it is important to consider the selection of machine elements and how to use them effectively. Machine elements include guides, power transmission mechanisms (ball screws, belts, chains), bearings, shaft couplings, springs, and seals. By properly combining these elements, it is possible to realize the optimum machine that satisfies the required specifications.
So how do you make the right choice? To do so, three elements are important:
(1) Accurately understand the required specifications (Quality)
(2) Keep within budget (Cost)
(3)Consider the process and delivery date (Delivery)
None of these elements are essential, and even one lacking is not a good design. These are also known as QCD, and are often used at manufacturing sites, but they should always be kept in mind at the design stage as well. In the following, we will consider these three elements in detail.
The most important thing is to understand the first requirement correctly. We need to decide whether this specification is a “must” or a “want”. If so, the design goals and planning specifications should be considered further. On the other hand, if it is a desirable requirement, it is necessary to check the range of essential elements and reflect it in the design. As some of you may have noticed, we’re not only talking about performance, but we’re also talking about cost. Performance, cost, and delivery are closely related, so it’s important to consider them holistically.
The second requirement is to stay within the budget. For example, for linear guides, consider other guide methods such as linear shafts, ball bushings, and cam followers in addition to linear guides, after considering the aforementioned performance aspects. can do. In drive transmission elements, the price fluctuates by using precision screws (ground) and rolled screws instead of ball screws. Induction motors, servo motors, air drives, etc. are available as drive elements (drive sources). The most cost-effective selection is made in consideration of performance and maintainability.
The third required specification, process and delivery, means that even if a superior machine is built, its value will decline if it cannot be delivered when it is needed. For example, consider going out to eat on your lunch break. Your lunch break is limited and you don’t have time to eat slowly. However, if you have to wait 20 or 30 minutes after ordering to be able to eat, your lunch break may be over. In such cases, no matter how affordable and delicious the food is, it is not available at lunchtime. Performance and cost exist, but delivery time is not considered.
When it comes to mechanical elements, issues such as ball screws often cause delivery problems. Precision ballscrews have high accuracy, but delivery times can be long. Advance arrangements should also be considered if essential. It is also possible that parts may not arrive when assembly begins. This can lead to delays and the need to change the process, which also affects costs.
As mentioned above, the selection of mechanical elements has a great influence on the design. When thinking about mechanical design, the main focus may be on the shape of the product and the associated costs, but the selection of machine elements is also very important. Read this magazine and work on improving the quality of your designs.
In the following, we will discuss the main mechanical elements in detail.
Linear selection guide: Appropriate selection method between radial type and four-way equal load type
Various products of linear guides are sold by various domestic and foreign manufacturers. In the past, it was said that the products of foreign manufacturers had quality problems, but now the quality has reached the same level as that of domestic manufacturers. Linear guide manufacturers also set up large-scale booths at exhibitions, making it clear that they are focusing on sales.
The structure of a linear guide is such that balls or rollers roll between blocks and rails. This creates rolling friction, resulting in a low coefficient of friction and high speed operation. Balls and rollers are designed to circulate inside the block, and as a result, the stroke of the block is infinite.
In terms of specifications, there are two main types of load bearing directions. Radial type and four-way equal load type. The radial type can receive a vertically downward load with the rail facing down. It can be used in a wall-mounted or ceiling-mounted position, but the permissible load for the reverse radial load and lateral load shown in Fig. 1 is relatively small. The four-way equal load type can receive the same amount of load in any orientation, such as radial direction, wall hanging, ceiling hanging, etc. If the load is mostly in the radial direction, the radial type should be selected, but if there is a lateral load, or if the load is to be hung on a wall or ceiling, the four-way equal load type should be selected.
Also, if a moment load is generated, use two blocks and widen the pitch to increase the allowable load. Details on how to calculate the allowable load are provided in the linear guide catalogs and technical materials, so please refer to them as necessary.
Ball screw selection guide: Selecting between precision screws and rolled screws
Like linear guides, ball screws are also sold by domestic and overseas manufacturers. There are also JIS standards for shaft diameters and leads, and there is compatibility among companies. However, care must be taken because there is almost no compatibility with the shape of the shaft end on the motor mounting side.
The structure of a ball screw is similar to that of a linear guide, with balls rolling between the screw shaft and the nut, and endlessly circulating inside the nut. Precise machining of the grooves in which the balls roll and ball size management enable positioning accuracy that cannot be obtained with slide screws.
A ball screw is an element that is frequently considered when selecting mechanical elements. The combination of feed speed (lead), shaft diameter (critical speed), and positioning accuracy (accuracy class) will be examined while referring to the catalog. Among these, the third accuracy class affects the ball screw price and delivery time.
Source: From THK technical data
https://tech.thk.com/en/products/pdf/en_b01_056.pdf
As shown in Table 1, accuracy grades range from C0 to C10, where C0 to C5 are ground threads (ground threads) and C7 to C10 are rolled threads. In general, ground screws are often called precision ball screws. For ground screws, the raceway surface on which the balls roll is finished by grinding. Because it is finished by grinding, it becomes a highly accurate screw, but it takes a long time to deliver and cost. It is important to do a rough review and confirm the delivery date with the manufacturer for the model number. If you proceed without confirming the delivery date after detailed consideration, it will take more time to rework. Time is of the essence, as most jobs have tight deadlines.
Roughly speaking, precision ball screws are used for positioning and rolled ball screws are used for transportation. The movement speed is determined by the combination of screw shaft rotation speed and lead, but if the shaft diameter is thin and long, it may exceed the critical speed (critical rotation speed). In that case, it is necessary to consider countermeasures such as changing the lead, increasing the shaft diameter, or giving up the use of the ball screw.
Source: From THK technical data
https://tech.thk.com/en/products/pdf/en_a15_011.pdf
Timing belt selection guide: Selection points and application examples according to accuracy requirements and stroke length
A timing belt is used when the accuracy of a ball screw is not required and the stroke is too long to use a ball screw. There are two types of timing belts, one for transportation and one for drive transmission. The material is rubber, and it contains steel, stainless steel, and Kevlar cords, so that it can be applied with high tension.
The trapezoidal tooth profile is for transportation, and the round tooth profile is for drive transmission (see Table 2). You can judge the use by distinguishing this by appearance. A small amount of torque can be transmitted even with a trapezoidal tooth profile, but if a large amount of torque is applied, there is a possibility of “jumping”, where the teeth are out of alignment with the timing pulley. This phenomenon is caused by the tooth profile and the allowable tension of the belt. On the other hand, for drive transmission, the allowable tension is high and the initial tension can be increased, reducing the possibility of tooth skipping.
| tooth profile | model |
|---|---|
| trapezoid | L、H、T |
| circular | S○〇M、MA、PX |
When using flex-type (seamless) belts, belts of standard lengths (lengths listed in the catalog) are available with relatively short lead times, but lengths outside the catalog range In the case of , the delivery time is generally longer. If you need a long belt, it is important to check the delivery date early, just like with ball screws.
The timing belt should be properly tensioned before use. Belt tensions may vary, so be sure to check your technical documentation and adjust the tension based on the length of belt you are using. Another thing we tend to forget is how to attach the belt. The timing pulley usually has a flange attached to it, and the belt must pass through the circumference of this flange. In other words, it is necessary to reduce the center spacing from the designed center spacing (belt spacing during use) to a position where the belt can be attached. Pay attention to proper tension adjustment and center-to-center shrinkage.
Roller chain selection guide: basic knowledge and practical use
Roller chain seems to be commonly used in industrial applications. Chains are used in combination with sprockets. If it is a JIS standard chain, it is compatible with different manufacturers and can be replaced without any problem. The size of the chain has notation such as “#40” (40th), “#50” (50th), “#80” (80th). The larger the size, the longer the pitch and the wider. For example, “#80” has a pitch of 25.4 mm (1 inch), “#40” has a pitch of 40/80 x 25.4 = 12.7 mm, and “#160” has a pitch of 160/80 x 25.4 = 50.8 mm. increase. Other sizes are determined by proportional calculation based on “#80”. It is useful to remember that “#80 has a pitch of 25.4 and the others are proportional calculations”.
As with timing belts, there are two types of chains, one for transportation and one for drive transmission. The above content was mainly related to drive transmission chains, but in the case of transportation, there are chains with a long pitch, chains with an accumulation function, and chains that can be transported at twice the chain speed. Since the chain is made of metal, it has little stretch and is suitable for carrying heavy loads. It also has high heat resistance, so it is sometimes used in and around furnaces. Chains and timing belts have similar properties, but use them according to the application.
Bearing Operating Conditions and Life Settings: Precautions for Ball Bearings
Ball bearings are widely used among bearings. Ball bearings are characterized by low friction and high speed rotation, but when selecting a ball bearing, it is important to accurately evaluate the direction and magnitude of the load. Incorrect evaluation of these can lead to bearing damage. Check whether the direction of load is in the radial direction only or whether the axial (thrust) direction is also included. If the load is in the radial direction only, a deep groove ball bearing (radial bearing) is suitable. However, when an axial load is applied, it is necessary to consider angular bearings. Also, the size of the load affects the life of the bearing. Consider the operating conditions when deciding how long to set the service life (replacement frequency) of the bearing.
In bearings, attention must also be paid to lubrication (greasing) and sealing. Deep groove ball bearings have contact and non-contact seals, but thrust and angular bearings have no seals, so sealing and lubrication must be considered.
As a precaution regarding the operating conditions of ball bearings, if they are used in locations subject to minute vibrations, the rolling elements (balls) will reciprocate on the spot, causing localized wear of the inner and outer rings. Under such conditions, wear progresses faster than the calculated life, so use under such conditions should be avoided. Please use it under the condition that the rolling element rotates at least once. If the rolling elements are changed from balls to needles (needle rollers), the distance for one rotation of the rolling elements will be shortened.
Gear types and selection points: How to select gears focusing on shape, material, finish, and accuracy
Gears are very important parts for power transmission. However, it is necessary to consider various specifications such as shape (combination direction of shafts), material, finish, accuracy, etc. Gear manufacturers offer a wide variety of gears based on combinations of these specifications, so if you can choose the right one from the lineup, it is worth considering purchasing.
The shape (combination direction of axes) can be parallel (spur gear), right angle (bevel gear), torsion, etc. We will select appropriate gears according to the arrangement of the axes. In addition, there are right angle and helical tooth shapes, so select these. Helical gears generate a thrust force against the shaft, but are characterized by smooth rotation. Especially at low speed rotation, the difference becomes clearer, so it is recommended to consider it when low speed and smooth rotation is required.
Hard steel (S45C, etc.), stainless steel (SUS304), and resin (MC nylon) are generally used as materials for gears. Tooth hardening is performed only on hard steel. Please select appropriately according to the usage environment.
Finish and precision go hand in hand. There are two types of gears: “cut finish” and “grind finish”. The cutting finish is completed by cutting using a hobbing machine. Grinding is finished by grinding with a whetstone after processing with a hobbing machine. Of course, the grinding process can be finished with higher precision. Precision gears have an exact tooth pitch, which means that the distance traveled per revolution is accurate.
Looking at the first picture in the catalog, I was able to choose “use this gear”. For example, it is a specific selection such as “Cutting finish with S45C material spur gear”. Next, consider the transmission torque, module, tooth surface strength, and bending strength to determine the gear. Regarding the tooth surface strength and bending strength, the calculation procedures are described in the manufacturer’s technical data and textbooks, so please refer to them.
Also, there is another important point in using gears. Be sure to provide a mechanism that enables backlash adjustment. If the backlash is small, it will not rotate, and if it is large, it will rattle. The optimal range is narrow and adjustments are delicate. Keep in mind that without the backlash adjustment mechanism it will not work properly.
Finally, there are CP (Circular Pitch) gears as special gears that are rarely mentioned in textbooks. CP gears are used only in the rack and pinion system, but these gears are designed so that the pitch of the rack (the distance between crests) is almost an integer. When using rack and pinion for positioning, we recommend using CP gears.
Shaft coupling selection guide: Consider coupling characteristics to absorb eccentricity and angular misalignment
Shaft couplings are used in shaft connections and are an important element that absorbs eccentricity and angular deflection. In the market, products are sold under the name of “coupling”. Many manufacturers offer different types of couplings.
Structurally, it consists of a hub that connects to the shaft and an intermediate part that absorbs eccentricity and deflection. Coupling characteristics appear due to differences in the structure of the intermediate part.
- Those with a wide or narrow allowable range of eccentricity and declination
- Those with large or small backlash
- Those with high or low allowable rotation speed
For conveyors that rotate in one direction, the effect of backlash is small, so chain couplings and gear couplings are often used because they have a wide tolerance for eccentricity and angular deviation even when backlash is large. On the other hand, when using a servomotor for positioning, a coupling with no or low backlash is chosen. Leaf spring couplings are sometimes used.
Since the transmission torque is determined by the size (outer diameter) of the coupling, refer to the allowable transmission torque in the catalog when selecting.
Clutch/brake selection guide: Detailed consideration of operating conditions and time charts
When selecting mechanical elements, it is common to use clutches and brakes that are sold as a unit. When making a selection, carefully plan the operating conditions (rotational speed, torque, clutch/brake operating time) and carefully consider the time chart. Then browse the catalog and make the appropriate selection.
Manufacturers’ websites and other sources provide information on selection, but they may not exactly match your requirements. It is important that you plan for yourself before referring to this information. If you can’t find a perfect match for your selection and need more information, we encourage you to contact the manufacturer’s contact point or your distributor’s sales representative.
Spring Design and Selection Guide: Collaborating with Manufacturer Expertise Is Important
Since spring manufacturing requires a high degree of expertise, we are limited in completely designing ourselves. The design of tension springs is particularly difficult. As much as possible, it is desirable to select products that the manufacturer has a lineup of. This ensures stable quality and smooth supply of replacement parts.
Compression springs are normally compressed slightly from their free length to apply a “set load” when in use. This serves two purposes. First, it prevents the spring from coming off the spring seat during operation. Second, to apply a preload. Automobile suspension springs do not come off because a set load is applied. Remember to apply a set load when using compression springs.
Tension springs are normally closed without load. This close contact state refers to the state in which the initial load is applied in the contraction direction. If we focus only on compression springs, we tend to forget about the initial load of tension springs. Due to the preload, the spring will not stretch beyond the preload when pulled. In the case of made-to-order, if you do not confirm the initial load with the manufacturer, you may encounter the problem of “something is wrong” after installing the device. The reason why I say that the design of tension springs is difficult is because of this preload. Regarding the initial load of the tension spring, it is ideal to consult with the manufacturer from the design stage, as it is necessary to consider manufacturing conditions.
Spring design rarely follows textbooks. Springs are highly specialized mechanical elements, and it is advisable to obtain the knowledge and cooperation of the manufacturer (but don’t rely entirely on them, but be actively involved).
How to choose and handle seals: The importance of fluid confirmation and design criteria
In general, O-rings and oil seals are the most used seals. Both of these are stipulated by JIS standards, and there are no problems with dimensions and availability (general materials). There are two points to note. The first is confirmation of the fluid to be used, and the second is the handling method.
Regarding the fluid used, since the material of the seal is rubber, it may be affected by some fluids. Be sure to check the catalog, which indicates the materials suitable for each fluid. Also, be careful when using it at high or low temperatures. As for the seal material, the manufacturer has knowledge about chemical substances, so if you have any questions, you can get an answer by contacting us.
Among mechanical elements, very careful handling is required. You might think of this element as a “little piece of rubber”, but it’s actually this little piece that prevents leaks. Air cylinders run smoothly and cars run clean with engine oil thanks to the use of the correct seals. At the design stage, it is necessary to strictly adhere to the housing (shaft and groove) design standards. These criteria are detailed in the catalog. Surface roughness, taper angle, dimensional tolerance, etc. are specified.
When assembling, wash carefully not to damage the seal, and assemble while lubricating. In particular, since the oil seal is inserted into the shaft, the seal surface rubs against the shaft as it is inserted. At this time, if there are scratches or dust on the shaft, scratches called “vertical scratches” enter the seal surface and cause leakage. Assembly work is done by the person in charge of assembly, but it is important to keep in mind the design that minimizes the possibility of scratches.
article author
Koji Professional Engineer Office
Koji Masakazu
https://www.pe-kouji.com/