Frequently Asked Questions

1. Why should I use STOBER ServoFit™ Precision Planetary Gearheads?

2. What do service reports show for life of units in industrial applications?

3. Since the steel motor shaft and the gearhead aluminum housing have different expansion rates,how do we prevent inducing thrust loads into the motor bearings and thus, reducing the motor's life?

4. How well can STOBER gearheads accept motors made to servo motor industry "standards"?

5. Why does STOBER use ductile iron housing?

6. What is the STOBER warranty?

7. How long will the gearhead maintain the measured backlash?

8. Why is the rating of 1.5 % of nominal torque used to check backlash?

1. Answer:

STOBER units:

• Run much cooler than the competitors. In applications with high speeds and rapid accelerations common to servo motors, STOBER units often have a skin temperature 40°C lower than our competitors. This is because of precision design features seal diameters, housing material ,and synthetic oil
• Have lowest standard backlash of any standard gearhead.
• Readily available stock units in 3 days or less.

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2. Answer:

Our experience in industrial applications is, that bearing failure is the most common mode of failure. However, our records confirm that our gearheads are exceeding our design criteria, which is a bearing "L10" life of 20,000 hours.

Our design achieved this proven life by focusing on:

• State of the art helical gearing for lower noise
  • HeliCamber™ gear technology - this method of gear cutting involves cambering the tooth profile and crowning the lead of the tooth to give the highest possible tooth contact.
  • Sun gear and planets are made from 16MnCr5 steel. They are case hardened to 61 Rockwell C and ground to a quality class 4 - 6 (Approx. AGMA class 11-13). The ring gear is heat treated ductile iron with a tensile strength of 700 - 900 N/mm² and cut to quality class 5 - 6 (Approx. AGMA class 11-12). This eliminates any irregularities that might generate heat, noise and pulsations in the drive train which can lead to speed fluctuation.
• Bearings
  • All loads are mounted between the bearings that support them. This straddle mounting results in more exact alignment which improves engagement of gears and lower heat and noise creation. The increase in distance between the bearings also provides high resistance to tilting to minimize gear tooth misalignment under radial force on the output shaft.
  • Input shaft bearing is outside the oil space. It has 2 shields and high temperature grease. This allows for a smaller diameter seal and greatly reduces heat generated. High temperature grease prevents burn or coking due to heat from motor and input seal drag.
  • Output Bearing has one shield. When the gearhead is mounted vertically, with output shaft up, even if operated at low speed, the lubrication of this bearing is assured . Under normal conditions grease is washed out by oil splashing over the bearing
• Shaft Seals contribute to long life
  • Care has been taken to minimize seal contact diameter.
    • Reduction of seal contact diameter by 1 mm reduces seal contact circumference by 3.14 mm.
    • Speed shaft moves past seal lip=revolutions * circumference
    • Seal contact area=width * circumference
    • Smaller seal=less speed and less contact area
    • Less surface area and less speed mean LESS HEAT
  • Double lip construction - high protection against leakage of oil and intrusion of dust, dirt, and moisture.
  • FKM seals for heat resistance allow continuous duty and high speed applications without concern.
• Single piece planet carrier and output shaft
  • Same casting supports both sides of planets with two bearing supports
    • Highest stiffness
    • Robust design
    • Accurate positioning of planets
  • Made from Ductile Cast Iron (GGG50)
    • High accuracy
    • No deformations due to heat
    • Tensile Strength 500 - 700 N/mm² guards against breakage of output shaft
  • Balanced
    • Low vibration
    • Smooth running
    • Center of carrier positioned to center distances 0.010 mm
      • Low backlash through accuracy, not with pretension. Thus, pre-load on the bearings is minimized
• Geared Housing
  • Ductile cast iron housing
    • Ductile iron (GGG70) - Steel like strength
    • High backlash stability
    • Long lifetime
    • High noise damping
    • Good short term lubrication characteristics in emergency condition of too little or no oil
  • Manufacturing method results in
    • Heat treating relieves stress in housing
    • Ring gear cut relative to same reference point used to cut the bearing seats results in high precision, low misalignment, low runout errors
    • High temperature stability with no deformation due to temperature
• Lubrication
  • Unit requires the same amount of lubricant regardless of mounting position, except PKX and PHKX.
  • Correct amount of synthetic lubricant installed at factory assures all of gear system is well lubricated over entire operating profile.
  • Air space over oil optimized to assure low over pressure as temperature rises
  • Lower performance loss than units that are grease packed
  • ISO VG 150
    • To withstand highest pressures between the tooth flanks - oil film does not pull off, so we have no metal to metal contact
    • Has low reduction of oil viscosity with temperature rise
    • Corrosion protection additives
  • Lifetime lubrication - no oil change required because:
    • Low heat creation means longer oil life
    • Sealed system
  • Low friction additives - increase efficiency
• TriAdapt™ motor shaft
  • Triple split collet
    • Works on same principle as a machine tool chuck
      • Motor shaft is clamped in exact center of collet
    • Aluminum Clamp Ring with single saw cut and one clamping screw
      • Low inertia
      • High input speeds possible
      • Balanced to assure no vibrations and smooth running
    • Customer needs only a torque wrench to attach motor
      • STOBER provides wrench with a ½" drive to fit collet clamp bolt
      • Customer uses torque wrench to tighten clamp bolt.
    • Adapter bushings
      • Adapt to all motor shaft diameters smaller than collet
    • Aluminum input cover
      • Lightweight design
      • O-Ring to seal between ductile cast iron housing and motor input cover has be added to units produced spring 1998 and later.
      • Adapt to new motor in short lead time
    • Aluminum motor plate
      • Lightweight design
      • Disassembling, to change motor unit is adapted for, is easy because oil cavity of reducer does not need to be opened
      • Adapt to new motor in short lead time

Our precise construction features an emphasis on minimizing seal contact diameters, resulting in a cooler and quieter running precise gearhead. A commonly accepted rule of thumb for lubricants states that, above 40°C, each 10°C reduction in oil temperature doubles the life a lubricant. This rule of thumb is proved by the excellent life that cool running STOBER planetary units are delivering around the world.

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3. Answer:

The distance between the inner race of the bearing supporting the sun gear and the inner-race of the motor's fixed bearing are fixed by the length of the steel shaft connecting them. (We'll call this the shaft distance). The distance between the outer-race of the bearing supporting the sun gear and the outer-race of the motor's fixed bearing are fixed by the length of the aluminum housing connecting them. (We'll call this the housing distance). (Motors have this same problem. Their shaft is steel and their housing is aluminum. They solve this by using a fixed bearing and an expansion bearing. The distance of material that will expand at differing rates is less if the fixed bearing is on the end nearest the gearhead.)

1 mm of aluminum is heated 1C, it expands 23.6 x 10-6 mm

1 mm of steel is heated 1 C it expands 11.0 x 10-6 mm

Thus the housing and shaft distances will diverge by difference in the expansion rate. (23.6 - 11=12.6). The housing will grow 12.6 x 10-6 mm farther than the shaft per mm original distance, per degree C temperature change. Because the housing is growing faster than the shaft, the trend will be, to pull our bearing toward the motor.

When we slide the motor shaft into the TriAdapt bushing, we will tend to move the inner-race of our bearing into the reducer as far as possible. When we tighten the clamping ring we will indeed make a rigid and fixed connection between the motor shaft and the TriAdapt bushing. As the system heats up, the inner-race of the gearhead will move toward the motor.

The STOBER high speed bearing has a C3 fit. The bearing in a P3 high speed cover has a minimum of 130 m (0.130 mm) axial internal clearance. Thus, we can move the inner-race this far before we consume this allowance. This 0.13 is a worst case figure for the axial internal clearance of the P3's high speed bearing. It is the bottom of the range (0.13- 0.28 mm) and is the range for the bearings in the smallest planetary gearhead. This allowance grows as the size of the gearhead grows. In addition, we are assuming that there is no clearance in the motor bearing. In fact, the motor must also have internal clearances in its bearings and sliding the motor shaft into the TriAdapt will tend to push the motor bearing's inner-race away from the reducer. The allowance in the motor bearings add a substantial safety margin to compensate for any clearance not used in the reducer bearing and other factors that may not be foreseen.

0.13 mm min allowance / 0.0000126 mm per C per mm length=10317.5 C mm shaft distance

0.28 mm max allowance / 0.0000126 mm per C per mm length=22222.2 C mm shaft distance

So if we anticipate a 40°C temperature rise in combination with a "shaft distance" of 257.9 mm. will consume the 0.13 mm minimum allowance of our bearing. Or a "shaft distance" of 555.5 mm. will consume the 0.28 mm maximum allowance of our bearing in a P3.

Repeating the process for a P8

0.23 mm min allowance / 0.0000126 mm per C per mm length=18253.9 C mm shaft distance

0.43 mm max allowance / 0.0000126 mm per C per mm length=34127.0 C mm shaft distance

So if we anticipate a 40°C temperature rise in combination with a "shaft distance" of 456.3 mm. will consume the 0.23 mm minimum allowance of our bearing. Or a "shaft distance" of 853.1 mm. will consume the 0.043 mm minimum allowance of our bearing in a P3.

In addition there is a small axial clearance in the bearing seat that also allows for expansion. In combination the axial clearance in our gearhead's bearing seat, plus the axial clearance in the gearheads bearings, plus the axial clearance the motor bearing system, result in sufficient clearance to prevent bearing failures due to heat generated thrust load. In part, the lack of problems we experience is because of the smaller seal contact diameters and other features described above, which result in a significant reduction in the heat generated by our gearhead. This reduction in heat is our most significant method of addressing the problems of thermal expansion. Heat not generated does not cause expansion and this expansion does not need to be dealt with.

Theoretical discussions such as this can be carried on with great fervor, but the true proof of the thesis is the real world. We believe that STOBER's five year warranty and our customer's experience with our gearheads in the real world, are the best proof that:

• our gearheads give long trouble free service
• do not have or cause problems due to thrust loads generated by thermal expansion.

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4. Answer:

German industry has two commonly accepted standards for servo motors. The two are DIN 42955 and DIN 42955R. The R suffix indicates that the second is a restricted standard, in that it requires the motor manufacturer to hold tighter standards. It is typical for the motor manufacturer to charge a premium for a motor made to this more exacting standard. STOBER designed the TriAdapt system to work with any motor that conforms to the DIN 42955 standard. By not requiring the tighter tolerances we reduce the total cost of a drive system using the STOBER gearhead. We adapt motors with more liberal standard by:

• A single split in the aluminum clamp ring assures centered and balanced clamping force, as opposed to competitor with two clamping screws that tend to move center of clamping force and balance away from center of collet.
• Triple split collet so the motor shaft will be clamped at the exact center of our collet.

As with the thermal question, the proof of this explanation will come when you run a STOBER gearhead.

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5. Answer:

Ductile iron has steel like strength [ 700 N/mm² (101,000 PSI)] allowing us to machine bearing seats and ring gear concentric within 0.010 mm. The ring gear is cut to Quality Class 5- 6 (approx. AGMA Gear Class 11-12)

Housing, ring gear, planet carrier and planet and sun gears all have very similar coefficient of thermal expansion - resulting in gear train geometry being stable across the entire temperature range.

Some have expressed doubt that STOBER can be assured of a reliable source of this high quality material. STOBER's manufacturing operation has received ISO 9001 certification. Quality control of inbound materials is an issue that we take very seriously. When I relayed this question to my German co-workers, their response was that they have used this quality of casting for years and over that time have built a network of vendors that can reliably deliver GGG70 quality ductile iron housings.

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6. Answer:

STOBER ServoFit products shall be free from defects in material and workmanship for a maximum of 5 years. All normal wear items, including oil seals and bearings, shall be covered for a peroid of 2 years. This warranty is the best in the industry.

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7. Answer:

Each gearhead is tested before the oil is added to assure that no unit has more than 3 arc minutes of backlash. When the oil is added to the gearhead, the backlash of the gear train is reduced by the oil film thickness. STOBER's HeliCamber™ gear technology reduces backlash degradation because cambering the tooth profile and crowning the lead of the tooth, increases the tooth contact surface.

We have tested our backlash over 106 cycles and charted the plot of Umkehrspanne and "lost motion" over the same 106 cycles. (Umkehrspanne is the plotted value of the difference between the error of the transmitted motion when the input shaft is rotated clockwise and counterclockwise.) Tests for torsional rigidity result in a Hysteresis. Lost motion measures the distance from the top path, "the loading" curve, to the bottom "unloading" curve. It indicated how precisely one can predict the position of the output shaft under load as it is influenced by torsional rigidity and backlash. Our tests demonstrate that our precision is stable over time.

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8. Answer:

The torque applied to check backlash is to overcome the static friction required to break the seals loose and move the planet carrier so that the flanks of the gears are in contact. Then by applying the same force in the opposite direction we move the gears to the point that flanks contact. The arc that the output shaft moves through is the backlash.

We discussed how STOBER minimizes the diameter of the seal contact, to the linear speed that the shaft moves past the seal lip and how it reduced heat generation. This minimization of contact diameter also reduces the torque required to move the shaft past the seal contact lip.

We have no problem moving our tooth flanks into contact with 1.5% of nominal torque. It is more than enough to overcome the seal drag and other static drag and move our gear flanks into contact. Once the teeth flanks are in contact, torsional rigidity of the gearhead is being tested.

If you have needed more torque in the past to overcome seal drag, it may be an indication of only one of the advantages of making minimum seal diameter an important design criteria.

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