In certain applications, the limited space available for the bearings and the loads to be supported require the use of a complete set of needles independent of a restraint system. The dimensions of the needle, diameter, and length, are determined according to the required load capacity.
The needles are inserted directly between the shaft and the housing, without using inner or outer rings. Therefore, a maximum diameter shaft is allowed to increase rigidity and load carrying capacity.
In rotating applications where the load carrying capacity requires the use of needles that are long in relation to the shaft diameter, it is preferable to use two rows of needles of the same length separated by a spacer ring. In such cases, needles with diameters in the same tolerance class must be selected. This arrangement is particularly recommended for mounting parts such as long idler gears, especially if they are subjected to torque.
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Maximum load capacity is achieved with hardened inner and outer raceways with a surface hardness of 58-64 HRC. The parts used to laterally support the needles at their ends should have equivalent hardness.
The inner and outer raceways should be aligned during installation and before operation under load. For parts equipped with a single row of needles, the inner raceway may be ground convex to allow for misalignment.
Convexity that allows misalignment of 1:1000 (or up to 2:1000 for instantaneous overload) does not reduce the calculated load carrying capacity. This convexity, which also depends on the length of the needles, can be produced on a separate inner ring or directly on the shaft journal using a grinding wheel with a concave profile, created by inclining the diamond impregnated cut-off wheel. Further technical information is available.
TYPES AND DIMENSIONS
The standard BR type needle, which is frequently used, has rounded ends. Needles with flat ends, type BP, can also be supplied on request. The standard dimensions of type BR needles are given in the following table (pages 198, 199 and 200). Needles with special dimensions can be manufactured on request. PROPERTIES
Standard needles are made of through hardened bearing steel with hardness 58-65 HRC. On request, needles can be made of heat treated corrosion resistant steel (hardness 57-62 HRC). The preferred diameters are 1.5, 2, 2.5, 3 and 4 mm. The surface finish is 0.2 micron according to the Ra system. The profile of a needle is not cylindrical over its entire length, as it tapers very slightly toward the ends. Therefore, an accurate measurement of the diameter can only be made in the middle area of the needle. Needles with a more pronounced taper at the ends can be supplied on request.
In general, the diameter of standard needles with rounded ends of type BR and with flat ends of type BP is manufactured with a tolerance of up to 10 microns less than the nominal dimension.
However, the maximum deviation in a production batch is 5 μm corresponding to one of the classes of grade G5 in the table below. On request, a deviation of 3 μm according to the classes of grade G3 and a deviation of 2 μm according to the classes of grade G2 can be obtained. Unless otherwise specified, the delivery quantities are divided into different classes of grades G2, G3, or G5. However, the current supply is generally available in class G2 according to the classes printed in bold.
The color codes specified for class G2 are used only by agreement. The length of BR and BP type needles is kept within the tolerance h13.
INSTALLATION OF LOOSE NEEDLES
Due to the large number of possible shaft diameters, which depend on the number of needles selected and their diameters, the needles cannot be packed in ready-to-install rings.
The needles supplied loose must therefore be arranged in a ring around the inner or outer raceway, which must be pre-greased to ensure their hold during assembly of the parts they are to hold.
In cases where the shaft must be inserted blindly into a needle ring, it may be useful to hold the needles in their housing by means of a fixing shaft of the same length as the needles. This can then be pulled out when the shaft is inserted.
The arrangement of the needles in a ring can be done by hand if the number of installations is small. The use of automatic machines with high-speed rotary feeding should only be considered for production volumes large enough to support the high investment costs.
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