1、13FTM04 AGMA Technical Paper Best Practices for Gearbox Assembly and Disassembly By J. Bello, David Brown Gear Systems, USA Inc.2 13FTM04 Best Practices for Gearbox Assembly and Disassembly Jodi Bello, David Brown Gear Systems, USA Inc. The statements and opinions contained herein are those of the a
2、uthor and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract When industry is looking at the best ways to increase efficiency, reduce downtime and increase profitability, gearbox performance and reliability are key factors. In most appli
3、cations gearbox reliability is critical to the productivity of the overall plant operation. Repair is often required with a swift turn around, as down time is very expensive. Designing for repair, and writing effective repair procedures, can speed the service time, and provide a quality refurbishmen
4、t. Minimizing down time and extending service life will contribute significantly to achieving the lowest overall operation costs. The best practices listed below are proven, effective methods used to install and remove bearings, seals, gears, couplings and shafts within a gearbox. These techniques a
5、re not new, and are usually obtained by hard won experience. Collecting them in one location is an attempt to document the best practices and provide a reference for design engineers. Engineers write the procedures for assembly and disassembly, they also dictate to the rest of the design team the de
6、sign intent. Including features to facilitate disassembly, minimizes repair cycle time and helps to prevent damage to components that could radically compromise their design life or performance. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virg
7、inia 22314 September 2013 ISBN: 978-1-61481-061-2 3 13FTM04 Best Practices for Gearbox Assembly and Disassembly Jodi Bello, David Brown Gear Systems, USA Inc. Introduction When industry is looking at the best ways to increase efficiency, reduce downtime and increase profitability, gearbox performanc
8、e and reliability are key factors. In most applications gearbox reliability is critical to the productivity of the overall plant operation. Repair is often required with a swift turn around, as down time is very expensive. Designing for repair, and writing effective repair procedures, can speed the
9、service time, and provide a quality refurbishment. Minimizing down time and extending service life will contribute significantly to achieving the lowest overall operation costs. The best practices listed below are proven, effective methods used to install and remove bearings, seals, gears, couplings
10、 and shafts within a gearbox. These techniques are not new, and are usually obtained by hard won experience. Collecting them in one location is an attempt to document the best practices and provide a reference for design engineers. Engineers write the procedures for assembly and disassembly, they al
11、so dictate to the rest of the design team the design intent. Including features to facilitate disassembly, minimizes repair cycle time and helps to prevent damage to components that could radically compromise their design life or performance. Basic types of component and assembly interfaces First we
12、 should examine the basic methods of attachments. Figures 1 through 4 illustrate some basic diagrams for the different types of common connections. Figure 1. Straight bore Figure 2. Tapered bore Figure 3. Splined bore Figure 4. Keyed bore 4 13FTM04 Components that have sustained damage in operation
13、may not retain their original dimensions. The design intent of the fit will have to be determined to appropriately determine the values for the repaired component. There are technical documents for designing each of these types of fits. Please see the references for some of the relevant technical sp
14、ecifications for more detailed information. Each of these interfaces can be made with different types of fits, clearance or interference. To determine which fit type you have, calculate the fit using the equation 1. F dD (1) where F = maximum fit; d = smallest diameter of bore; D = the largest shaft
15、 diameter. Measure the bore and shaft at several locations, and use the smallest diameter bore and largest diameter shaft. If the shaft and bore tolerances are available, the entire expected fit range can be calculated. (To calculate the minimum fit, you would use the largest bore diameter minus the
16、 smallest shaft diameter.) If this value is positive, the fit is clearance, if it is negative, the fit is interference. If the value is zero the parts could theoretically slide together, but in practice a small amount of force or thermal difference is needed for assembly. The clearance value needed
17、to slide parts together easily is generally assumed to be at least 0.001 inches. For long fits and large diameters more clearance may be required, evaluating the tolerance and run out of the parts will help determine an appropriate value. Clearance fits Clearance fits are used for easy assembly, in
18、typically low speed applications. Set screws can be used to connect the shaft to the hub and transmit torque. Straight bore clearance fits slide together easily. There is no axial location control with this fit alone, and limited radial location. Shoulders, setscrews and pins can be used to control
19、axial locations. Splined connections fit multiple tooth internal teeth against external teeth. There is clearance on both the sides and diameters of the teeth. Keyways transmit the torque between the shaft and hub. Parts assemble easily. Setscrews can be used to fix the key and shaft in the bore. In
20、terference fits When assembled the bore expands and / or the shaft contracts so that the interface is in compression. Interference fits can transmit more torque than clearance fits. There are several different methods for assembly which will be discussed later. These fits are typically used to contr
21、ol location of the components, axial and radial, as well as transmit torque. Interference fits are also used to maintain balance of components in high speed applications. - Straight. A straight interface transmits torque while maintaining both axial and radial location control of the components. - T
22、apered. A tapered shaft and bore under compression can be used to transmit the friction torque. The compression can be obtained by drawing the shafts together using a shaft nut or by thermal differential assembly. - Keyways. Keys with interference fits do not shift and alter the balance of the compo
23、nents. They also can transmit more torque than a straight or tapered interface with the same interference, because the key helps ensure the joint will not slip. - Splines. Usually these are interference fit on the outside diameter of the splines. These are typically used when radial position needs t
24、o be controlled. - Bushings. There are various mechanical devices that can be used to create an interference fit. They slip on to the shaft with clearance, but when engaged create an interference fit between the shaft and hub. Tightening these devises is best done in a star pattern for proper center
25、ing of the parts. 5 13FTM04 Centering is especially important if balance is critical. The best practice here is to follow the individual manufacturers assembly instruction. - Transition fits. Transition fits can be either clearance or interference. The tolerance range on the parts can result in a sm
26、all interference or clearance based on the individual components. These fits are commonly used to ease assembly in applications that still require close fits. They can be assembled using the interference fit techniques but with much less force or temperature difference. - Bearing fits. Bearing assem
27、bly is a special case. Bearings may be interference fit on one race of the bearing and clearance on the other. The mounting of the bearing on the shaft and in the housing will determine the operating clearance of the bearing. Having the correct bearing fit for the application is critical to achievin
28、g the design life and reliability of a bearing. Bearing catalogs have more information or consult the manufacturers directly for each application. Assembly techniques Thermal differential Heating or cooling components can cause them to expand or contract to overcome interference and allow for easy a
29、ssembly. The amount of temperature difference required can be calculated by using equation 2 (use consistent units). Thermal differential (simplified equation) .Tdia(2) where = diametral interference; = material coefficient of thermal expansion; dia = diameter in question. Most coefficients are give
30、n at a specific temperature, and will give a close enough approximation to the change in temperature for assembly purposes. Add a few thousands of an inch to the diametral interference to give a resulting clearance for assembly after the heat has expanded the parts. It is common to add a few degrees
31、 to the delta to compensate for handling time and the assumptions of the equation. Rounding the value up by 20 degrees, or to an easy to measure value, is acceptable, as long as this does not put the value beyond the material limits. These material limits are based on composition and heat treatments
32、. It is important not to exceed these limits as this could impact the ability of the component to function properly. Because the temperature will change with time it is important to have all the fixtures and tools for assembly prepared before removing the component from the oven or freezer. As for a
33、ny job proper personal protective equipment should be worn as the parts will not be able to be touched by bare skin. If there is a question about the temperature of the components or the measurement system parts can be measured before assembly, at temperature, to determine they have reached the prop
34、er size. This must be done quickly because the delicate measurement instruments will be affected by the temperature too. It is easier to measure the temperature, but when developing a new process this technique can give valuable information. Use equation 2 to determine the amount of temperature diff
35、erence required. The temperature differential can be obtained by heating, cooling or a combination of both. Lifting holes or fixtures for holding the components are especially important in thermal differential assembly. The fixtures need to be able to withstand the temperature, and also be out of th
36、e way for assembly. Planning the methods for lifting must be done before the components have changed temperature. Having components level during assembly seems like a simple thing, but is often overlooked. It is easy to get components jammed if they are not aligned. Occasionally this can be overcome
37、 with gentle taps, but careful alignment can eliminate this need. 6 13FTM04 Components that have been assembled using heat or cold can creep apart as they cool. A bearing that is seated against a shoulder can move apart ever so slightly and this can impact operation later. Clamping the parts or gent
38、ly tapping a spacer down repeatedly as things normalize can prevent this. Never tap a bearing across the rollers. It is ok to gently tap an inner race seating on a shaft, or an outer race without rollers into the housing. Heating There are several methods of heating components, induction heaters, ov
39、ens, or hot oil baths. Thorough heating and consistent temperature is required. The best method is dependent mainly on economics and available resources. Ovens make sense for large parts, high volume production, or for time savings. Many ovens can run un-attended so parts can be loaded at the end of
40、 shift and heated overnight for assembly the next morning. This allows thorough heating and efficient use of time. Induction heaters are fast an efficient. Load the part, press the button and the heater runs. It monitors the temperature and shuts off when temperature is achieved. Most machines will
41、monitor the temperature and reheat the part if the temperature drops more than 5 degrees. Hot oil baths are a time proven solution for heating parts. However, careful monitoring is required to prevent the oil from catching fire and additional safety procedures must be observed to protect the operato
42、r from the hot oil. No matter what the method, care must be taken to prevent overheating of the parts. There are various methods available such as, infrared thermometers, contact thermometers, or even temple sticks, (wax crayons that melt at a specific temperature). Cooling Cooling can be done with
43、freezers, dry ice or liquid nitrogen. When using liquid nitrogen, use caution that freezing the components will not damage them. There are some heat treated components that should not be cryogenically treated. There is always a chance of condensation forming on frozen parts. Wiping them down with is
44、opropyl alcohol before assembly will help to dissipate the moisture. This should also be done as the parts return to ambient temperature if condensation appears. Freezers are very convenient because parts can be placed in the freezer overnight and assembled in the morning. Dry Ice can be packed arou
45、nd parts that need to be cooled. It is more difficult to get a consistent cooling of the parts due to it being solid. Use a thermometer to get an accurate temperature realizing that it is a surface temperature. Parts may need to soak for a considerable time to be cooled through. It is not often that
46、 both cooling and heating are required. This high amount of interference may better be obtained by pressing the parts together. There is a high risk of the parts cracking from thermal shock when heat and cold are both required. Press Parts can be pressed together using a mechanical or hydraulic pres
47、s. Caution must be taken when using a press, as the forces are very large and the process can be dangerous. As with all work, proper personal protective equipment and protective guarding around the equipment is recommended. Basic equation. See Figure 5 for visual depiction of press fit. FAP (3) A dL
48、 (4) 7 13FTM04 Figure 5. Press fit illustration 22 2222 22oioiPdd ddEEdd dd (5) where F = force to press; A = area of interface; = coefficient of friction; P = interface pressure; d = shaft diameter/bore diameter (nominal); L = length of fit; = diametral interference; do= hub outside diameter; di= b
49、ore in shaft or zero if solid shaft; Eo= Youngs Modulus of hub; Ei= Youngs Modulus of shaft; o= Poissons Ratio for the hub; i= Poissons Ratio for the shaft. The force calculated here is approximate and should be considered a minimum. It can be used to size the equipment needed. More force than calculated may be required if there is damage to the mating surfaces, or the parts are misaligned. Surface finish effects have not been considered in this calculation. The first operation before pressing should be leveling the par
