Hardness on the performance of HGR linear guide multi-dimensional impact
HGR linear guide (such as HIWIN HGR series) of the core load-bearing components (guide rail slide surface, slide raceway, ball) need to be heat-treated (such as the whole quenching + surface grinding) to achieve a specific hardness, and its hardness value (usually measured in Rockwell hardness HRC, the mainstream range of HRC58-62) impact on the performance of through the "load - wear - life - precision" full cycle. The impact of its hardness value (usually measured by Rockwell hardness HRC58-62) on the performance throughout the whole cycle of "Load - Wear - Lifespan - Accuracy" is as follows:
First, the decisive role of wear resistance
linear guide wear mainly from the rolling friction of the ball and slide / raceway and micro-impact (such as start-stop moment). Hardness directly determines the surface wear resistance:
When the hardness ≥ HRC58, the surface metal crystal structure is more dense, strong resistance to plastic deformation, the ball rolling is not easy to produce "furrow effect" (i.e., hard particles embedded in the soft surface of the formation of scratches). For example, under the same load (F=5000N) and travel (100km), the amount of wear on the surface of a guideway with HRC60 is only 1/3 that of HRC50 (approx. 0.5μm vs 1.6μm).
If the hardness is insufficient (e.g. below HRC55), the surface is prone to "pockmarks" or "depressions" after long-term operation, resulting in an increase in the coefficient of friction from 0.001 to 0.003 or more, leading to fluctuations in running resistance (±5% or more) and even vibration noise (>70dB). (>70dB).
Second, the bearing capacity and the impact of plastic deformation resistance
rail bearing capacity (especially dynamic load) depends on the compressive strength of the contact surface, and hardness and compressive strength is positively correlated (HRC every 1 level, the compressive strength increased by about 50MPa):
When the hardness reaches HRC60, the maximum contact stress that the contact surface of the guideway can withstand is about 2000MPa, which meets the dynamic load requirements of most industrial equipment (e.g. CNC machine tools) (usually dynamic load ≤ 1500MPa).
If the hardness is low (such as HRC50), under high loads (such as F = 10,000N), the slide and ball contact point will produce permanent plastic deformation (depression depth > 2μm), resulting in ball trajectory offset, which in turn makes the guide rail straightness error from the initial 10μm/1000mm increased to more than 30μm/1000mm.
Third, the significant impact on the contact fatigue life
rolling contact, the guide surface is subjected to cyclic alternating stress (frequency changes with the operating speed, such as 30m/min when about 50Hz), easy to produce contact fatigue (surface spalling, cracks). Hardness is the core index of resistance to contact fatigue:
When the hardness is in the range of HRC58-62, the fatigue limit of the material is high (about 1200MPa), and the number of cycles (life) that can be withstood can be up to 10^7-10^8 times (corresponding to a travel of about 1000km). Experimental data show that the contact fatigue life of HRC62 rails is 2-3 times higher than that of HRC55.
If the hardness exceeds HRC63 (such as HRC65), the material brittleness increases, and the surface is prone to "brittle spalling" due to small stress concentrations (such as machining residual scratches), instead of shortening the life expectancy (e.g. from 1000km to 600km).
Fourth, the long-term impact on precision retention
HGR guideway precision retention (i.e., long-term use of straightness, parallelism stability) depends on the surface deformation resistance, and hardness is the key:
High hardness (HRC60-62) surface in the long-term load is not easy to produce "creep" (slow plastic deformation), for example: in the sustained load 1000N, ambient temperature 25 ℃, 1000mm long rail length change ≤ 0.5μm / year, to ensure that the equipment machining accuracy (such as lathe roundness) long-term stability in the 0.005mm or less.
If the hardness is insufficient (e.g. HRC50), the guideway may bend more than 0.01mm after 1 year due to plastic deformation, resulting in the slider running "stuck" or "yaw", which directly affects the positioning accuracy of automation equipment (e.g. PCB drilling machine positioning). (e.g. PCB drilling machine positioning error increases from ±0.01mm to ±0.03mm).
Five, on the installation and fitness of the indirect impact of
too high hardness (such as HRC65 above) will lead to increased brittleness of the guide material, installation, if there is a slight impact (such as uneven bolt preload), may produce "hidden cracks"; and low hardness (HRC55 the following) may be in the installation of flat leveling due to uneven force resulting in localized concave The hardness of the HGR guideway is not uniform, but rather reduces the assembly precision. Therefore, the hardness of HGR guide rail should be controlled in HRC58-62 "toughness - hardness balance interval", taking into account the processability (such as grinding is not easy to chipping) and the use of reliability.
Summarize
The hardness of HGR linear guide is the core control index of "wear resistance - load carrying capacity - fatigue life - accuracy retention": too low will accelerate the wear, reduce the life and deteriorate the accuracy; too high will increase the risk of brittleness. In practice, the hardness level should be selected according to the working conditions (e.g. load, speed, amount of dust in the environment) (e.g. HRC60-62 for heavy-duty machine tools, HRC58-60 for light-duty automation equipment) and the performance should be ensured by hardness testing (e.g. micro-hardness tester sampling hardness of the surface at a depth of 30 μm).
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