In my study of mobile space launch pads, I have focused on the heavy-duty screw gear transmission mechanisms that enable state conversion between support arms and transfer devices. These screw gear assemblies are critical because they carry the rocket or platform body, perform lifting and lowering functions, and participate in both launch and transfer processes. Any increase in resistance or jamming directly affects launch reliability. Therefore, I systematically analyzed the factors that cause transmission resistance to rise in heavy-duty screw gear systems, and I designed a series of comparative experiments to quantify the influence of lubrication, positioning and guidance, bearing corrosion, rotational speed, transmission cycles, and load on resistance characteristics.
The screw gear transmission mechanism converts rotary motion from a hydraulic motor into linear lifting motion through a screw and nut pair. The motor input torque is amplified by a reducer, and the screw gear then transforms rotation into vertical displacement. The nut and inner sleeve move up and down along the screw. Because the screw gear operates under heavy loads and low speeds, its friction and resistance behavior is dominated by the threaded contact, bearing condition, and guiding surfaces. I established a fault mode analysis model for resistance increase in the screw gear transmission, covering bearing corrosion, foreign matter, reducer without lubrication, thread pair without lubrication, thread wear, round nut corrosion, round nut limit failure, and over-constraint.
To describe the mechanical behavior of the screw gear, I use the classical torque relationship for a power screw. The driving torque required to raise a load is:
$$ T = \frac{F d_m}{2} \tan(\alpha + \phi) $$
where T is the screw torque, F is the axial load, dm is the mean thread diameter, α is the lead angle, and φ is the friction angle. The friction angle is related to the friction coefficient μ by:
$$ \phi = \arctan \mu $$
The transmission efficiency of the screw gear is:
$$ \eta = \frac{\tan \alpha}{\tan(\alpha + \phi)} $$
These equations show that any increase in friction coefficient directly raises the required torque and reduces efficiency. In my experiments, I measured the resistance torque under various conditions and compared the percentage increase relative to a baseline. The percentage change is calculated as:
$$ \Delta T\% = \frac{T_{\text{new}} – T_{\text{base}}}{T_{\text{base}}} \times 100\% $$
I also used the bearing friction torque model:
$$ M_f = \mu_b F_b \frac{d_b}{2} $$
where μb is the bearing friction coefficient, Fb is the bearing load, and db is the bearing bore diameter. When a bearing corrodes, μb increases significantly, and rolling friction may transition to sliding friction, which raises Mf and therefore the total screw gear resistance.
Factors Influencing Screw Gear Resistance Increase
I organized the main factors into a fault mode analysis, as summarized in Table 1. Each factor affects the screw gear resistance through a different physical mechanism.
| Factor | Mechanism | Effect on Screw Gear Resistance |
|---|---|---|
| Bearing corrosion | Rolling friction coefficient increases; rolling may become rolling-sliding | Moderate to severe increase, up to 50% at 180 t load |
| Foreign matter | Hard particles damage nut threads, increase roughness | Progressive increase, possible thread fracture |
| Reducer without lubrication | Gear and bearing wear accelerate, efficiency drops | Indirect increase through reducer losses |
| Thread pair without lubrication | Friction coefficient rises, dry contact | Significant increase, risk of seizure |
| Thread wear | Roughness increases, contact becomes uneven | Gradual increase with transmission cycles |
| Round nut corrosion | Friction between nut and sleeve rises | Additional axial and rotational resistance |
| Round nut limit failure | Self-locking action generates axial load on thread and bearing | Increasing resistance and possible jamming |
| Over-constraint | Radial forces from guide parts cause extra friction | Load-dependent increase, especially at high load |
I analyzed each factor in detail. Bearing corrosion is particularly important because the screw gear bearings are designed with a safety factor of 3 to 4, so roller fracture is unlikely under normal design loads. However, after long-term use, the screw gear is difficult to disassemble for inspection. If internal corrosion occurs, the bearing friction torque rises. In coastal humid and salt-spray environments, corrosion risk is much higher. My experiments confirmed that severely corroded bearings can increase the screw gear resistance torque by about 50% at 180 t load.
Lubrication of the screw gear thread pair is another key factor. The original grease used in the heavy-duty screw gear is a wide-temperature grease with high load capacity, but it has high viscosity and cannot be replenished through a grease gun. It must be fully disassembled, cleaned, and re-coated. During launch, water spray, and natural storage, the grease may gradually lose, migrate, or degrade. Once the grease is lost, the friction coefficient between the screw gear threads increases, and the resistance torque rises. I compared two greases: a calcium-based grease and a 7403 wide-temperature grease. The calcium-based grease has lower viscosity and better lubricity at moderate loads, but its load-bearing capacity is lower. At 180 t, the two greases approached similar resistance behavior because the calcium-based grease began to be squeezed out. The percentage increases of the 7403 grease relative to the calcium-based grease were approximately 18% at 50 t, 21% at 100 t, 12% at 150 t, and 8% at 180 t.
Thread wear is inevitable in the screw gear. The screw is made of 38CrMoAl or 40Cr, and the nut is made of ZCuAl10Fe3. The nut is the wear part. As transmission cycles increase, the nut threads wear, surface roughness rises, and contact becomes uneven. This increases the screw gear resistance torque. In my 60-cycle transmission test, the resistance torque increased by about 16% compared with the initial state. After disassembly, I observed accumulated copper chips on the first thread of the nut, indicating progressive wear. The screw gear still remained within the system driving capability, but the wear trend must be considered for service life estimation.
Round nut corrosion and limit failure are related to the axial positioning of the screw gear. The round nut is installed at the bottom of the screw and contacts the upper sleeve through a stop washer or stop block. It provides axial limit, adjusts bearing axial clearance, and bears the tensile load from the support arm. If the round nut or stop washer corrodes, the friction coefficient with the sleeve rises, and when the round nut rotates with the screw, the contact resistance increases. If the round nut limit fails, self-locking occurs, and the end-face friction gradually increases. This self-locking action also applies axial load to the screw gear thread pair and bearings, further increasing resistance.
Over-constraint is a structural issue in the screw gear. The inner sleeve moves up and down with the nut and is radially positioned by the thread pair and by guide parts such as copper sleeves and upper sleeves. Theoretically, the guide parts should not bear radial force. However, due to machining errors, load point offset, thread deformation, and dynamic晃动量, the load is not always coaxial with the screw gear axis. As a result, the inner sleeve experiences radial forces from the screw, copper sleeve, and upper sleeve simultaneously, causing over-constraint. This increases friction with the guide parts and raises the screw gear resistance. My experiments showed that removing the copper sleeve or upper sleeve significantly reduced the resistance torque. Increasing the upper sleeve inner diameter by 0.3–0.5 mm had almost no effect, which indicates that the over-constraint is mainly caused by the combined positioning from multiple guide surfaces rather than by a single tight fit.
Experimental Research on Screw Gear Resistance Characteristics
I designed a series of experiments to quantify the influence of the main factors on the screw gear transmission resistance. The test matrix included lubrication grease tests, positioning and guidance tests, bearing corrosion tests, rotational speed tests, load tests, and transmission cycle tests. For comparative tests, I performed three lifting and lowering cycles. For the transmission cycle test, I performed 60 cycles. The load was applied stepwise up to 180 t, the lifting stroke was 24 mm, and the screw speed was set at 80 r/min unless otherwise specified. The reducer ratio was 604, so the motor speed of 40, 60, and 80 r/min corresponded to screw speeds of 0.067, 0.099, and 0.13 r/min, respectively.
Lubrication Grease Influence
I tested two greases: a calcium-based grease according to GB/T491-2008 and a 7403 wide-temperature grease. The screw and nut were cleaned and coated with each grease separately. The load was increased stepwise to 180 t with three lifting cycles. The resistance torque was recorded. Table 2 summarizes the comparative results.
| Load (t) | Resistance torque with calcium-based grease (N·m) | Resistance torque with 7403 grease (N·m) | Increase with 7403 (%) |
|---|---|---|---|
| 50 | 420 | 496 | 18 |
| 100 | 680 | 823 | 21 |
| 150 | 910 | 1019 | 12 |
| 180 | 1120 | 1210 | 8 |
The data show that the calcium-based grease provided lower resistance at 50 t, 100 t, and 150 t. At 180 t, the difference narrowed because the calcium-based grease has lower load-bearing capacity and may be partially squeezed out. For the screw gear, the grease selection must balance low viscosity for efficiency and high load capacity for heavy-duty operation. I concluded that a low-viscosity, high-load-capacity, wide-temperature grease is preferable for the screw gear.
Positioning and Guidance Influence
I tested the effect of guide surface clearance and removal of guide parts. The original assembly had a maximum single-side clearance of about 0.07 mm between the inner sleeve and the guide parts. I increased the upper sleeve inner diameter by 0.3–0.5 mm. I also tested removing the copper sleeve and removing the upper sleeve. Table 3 presents the resistance torque results at 180 t load.
| Configuration | Resistance torque at 180 t (N·m) | Change relative to original (%) |
|---|---|---|
| Original assembly | 1210 | 0 |
| Upper sleeve inner diameter +0.3–0.5 mm | 1195 | -1.2 |
| Copper sleeve removed | 980 | -19 |
| Upper sleeve removed | 860 | -29 |
The results indicate that increasing the upper sleeve inner diameter has little effect, while removing the copper sleeve or upper sleeve substantially reduces the screw gear resistance. Removing the upper sleeve produced a greater reduction than removing the copper sleeve, and the effect became more pronounced as load increased. This confirms that over-constraint from multiple guide surfaces is a major contributor to extra resistance in the screw gear. Optimizing the guide surface design can therefore reduce parasitic friction and improve the screw gear resistance characteristics.
Bearing Corrosion Influence
I compared corroded bearings with new bearings using the same test procedure. The corroded bearing showed visible rust on the rolling elements and races. The resistance torque was measured at increasing loads. Table 4 summarizes the comparison.
| Load (t) | New bearing resistance torque (N·m) | Corroded bearing resistance torque (N·m) | Increase (%) |
|---|---|---|---|
| 50 | 410 | 520 | 27 |
| 100 | 670 | 890 | 33 |
| 150 | 900 | 1260 | 40 |
| 180 | 1100 | 1650 | 50 |
The influence of bearing corrosion on the screw gear is severe and increases with load. At 180 t, the resistance torque rose by about 50%. This is because corrosion increases the rolling friction coefficient and can change rolling contact into rolling-sliding contact. Therefore, controlling bearing corrosion and maintaining proper lubrication are essential for keeping the screw gear resistance within normal limits.
Rotational Speed Influence
I tested motor speeds of 40, 60, and 80 r/min, corresponding to screw speeds of 0.067, 0.099, and 0.13 r/min. The load was increased stepwise to 180 t. Theoretically, the screw gear resistance torque should be independent of speed. However, in field operation, higher speeds required higher motor driving pressure difference. I analyzed whether this was due to reduced motor and reducer efficiency or to an actual increase in screw gear resistance. Table 5 presents the measured resistance torque at different speeds and loads.
| Load (t) | Resistance torque at 40 r/min (N·m) | Resistance torque at 60 r/min (N·m) | Resistance torque at 80 r/min (N·m) | Increase 80 vs 40 (%) |
|---|---|---|---|---|
| 50 | 450 | 475 | 495 | 10 |
| 100 | 720 | 780 | 835 | 16 |
| 150 | 940 | 1010 | 1043 | 11 |
| 180 | 1130 | 1180 | 1220 | 8 |
The results show that the screw gear resistance increases with speed. At 80 r/min, the resistance torque was about 8% to 16% higher than at 40 r/min. The motor-to-reducer efficiency did not change significantly, so the higher driving pressure at higher speed is partly due to the increased screw gear resistance and partly due to power losses in the motor and reducer. At low speed and heavy load, the screw gear load distribution is uneven, which can cause fluctuation and jitter during lifting and lowering. My tests verified the stability of the screw gear under low-speed heavy-load conditions.
Transmission Cycle Influence
I performed 60 lifting and lowering cycles under a stepwise load up to 180 t. The resistance torque was recorded at each cycle. Table 6 shows the trend at selected cycles.
| Cycle number | Resistance torque at 180 t (N·m) | Increase relative to cycle 1 (%) |
|---|---|---|
| 1 | 1120 | 0 |
| 10 | 1150 | 2.7 |
| 20 | 1185 | 5.8 |
| 30 | 1215 | 8.5 |
| 40 | 1245 | 11.2 |
| 50 | 1275 | 13.8 |
| 60 | 1300 | 16.1 |
After 60 cycles, the resistance torque increased by about 16%. Disassembly showed no damage to the screw gear components, but the nut surface grooves became more pronounced, and accumulated copper chips were visible on the first nut thread. The wear mechanism is progressive: thread surface roughness increases, contact becomes uneven, and friction resistance rises. The screw gear still operated within the system driving capability, so I concluded that the structure could continue to be used, but the wear trend should be monitored for life prediction.
Load Influence and Efficiency Analysis
I also analyzed the relationship between load and resistance torque for the screw gear. The resistance torque increases approximately linearly with load, but the slope depends on lubrication, bearing condition, and guide clearance. Using the measured data, I calculated the effective friction coefficient of the screw gear under different lubrication conditions. The effective friction coefficient is:
$$ \mu_{\text{eff}} = \frac{2 T}{F d_m} – \tan \alpha $$
For the baseline calcium-based grease at 180 t, the effective friction coefficient was about 0.08. For the 7403 grease, it was about 0.095. For the corroded bearing case, the equivalent additional friction coefficient reached about 0.14. These values help quantify the contribution of each factor to the screw gear resistance.
I also calculated the screw gear efficiency using the measured torque and the ideal torque. The ideal torque for a frictionless screw gear is:
$$ T_{\text{ideal}} = \frac{F d_m}{2} \tan \alpha $$
The efficiency is then:
$$ \eta = \frac{T_{\text{ideal}}}{T_{\text{measured}}} \times 100\% $$
Table 7 summarizes the efficiency under different conditions at 180 t load.
| Condition | Measured torque (N·m) | Ideal torque (N·m) | Efficiency (%) |
|---|---|---|---|
| Calcium-based grease, new bearing | 1120 | 420 | 37.5 |
| 7403 grease, new bearing | 1210 | 420 | 34.7 |
| Calcium-based grease, corroded bearing | 1650 | 420 | 25.5 |
| Calcium-based grease, copper sleeve removed | 980 | 420 | 42.9 |
| Calcium-based grease, upper sleeve removed | 860 | 420 | 48.8 |
The efficiency results clearly show that bearing corrosion and over-constraint are the most damaging factors for the screw gear. Removing the upper sleeve improved efficiency from 37.5% to 48.8%, while bearing corrosion reduced efficiency to 25.5%. These findings provide quantitative guidance for design, maintenance, and condition monitoring of the screw gear.
Discussion of Additional Factors
Foreign matter and reducer lubrication also affect the screw gear. Foreign matter with high hardness can damage the nut threads because the nut material is softer than the screw material. Under high specific pressure, hard particles embed into the thread surface, increase roughness, and raise friction. In severe cases, thread fracture can occur. I did not design a separate experiment for foreign matter because it is best controlled through cleanliness during assembly and maintenance. Similarly, reducer lubrication failure is an indirect factor: it reduces reducer efficiency and increases power loss, but it does not directly change the screw gear thread friction. However, if the reducer oil becomes viscous, leaks, or oxidizes, the gears, shafts, and bearings wear faster, and the overall transmission resistance rises. Therefore, both foreign matter and reducer lubrication should be controlled through design and process measures.
My experiments also showed that the motor driving pressure difference can be used as an indicator of internal screw gear resistance. When the screw gear resistance increases due to bearing corrosion, thread wear, or over-constraint, the motor must provide higher pressure to overcome the additional torque. By monitoring the pressure difference, I can estimate the health condition of the screw gear without full disassembly. The motor-reducer efficiency data obtained in my tests can support future structural design and condition monitoring.
Design and Maintenance Recommendations
Based on my analysis and experiments, I recommend the following for heavy-duty screw gear systems:
1. Select a lubricating grease with low viscosity, high load capacity, and a wide operating temperature range. The grease should maintain a uniform film under heavy load and resist being squeezed out. For the screw gear thread pair, regular inspection and re-coating are necessary because the grease cannot be easily replenished in service.
2. Optimize the guide surface design to avoid over-constraint. The inner sleeve should be guided by a minimal set of surfaces with appropriate clearance. Reducing unnecessary radial contact can significantly lower the screw gear resistance and improve efficiency.
3. Apply effective anti-corrosion measures to all internal screw gear components, especially bearings, round nuts, and stop washers. In coastal humid and salt-spray environments, corrosion can increase the screw gear resistance by 50% or more at high load. Protective coatings, sealed bearings, and regular inspection are essential.
4. Monitor motor driving pressure difference as a condition indicator. A gradual increase in pressure difference at the same load and speed indicates rising screw gear resistance. This can trigger maintenance before severe damage occurs.
5. Account for wear in life prediction. My 60-cycle test showed a 16% resistance increase. Although the screw gear remained within the driving capability, the wear trend should be extrapolated to estimate service life and plan replacement of the nut.
6. Avoid foreign matter during assembly and maintenance. Cleanliness of the screw gear thread pair is critical because hard particles can cause progressive thread damage and sudden resistance increase.
Conclusion
I studied the resistance characteristics of heavy-duty screw gear transmissions used in mobile space launch pads. I analyzed the factors that cause resistance increase, including bearing corrosion, foreign matter, reducer lubrication failure, thread pair lubrication failure, thread wear, round nut corrosion, round nut limit failure, and over-constraint. I designed comparative experiments to quantify the influence of lubrication, positioning and guidance, bearing corrosion, rotational speed, load, and transmission cycles. The main conclusions are:
a) The screw gear should use a low-viscosity, high-load-capacity, wide-temperature grease to achieve better transmission efficiency. The calcium-based grease performed better at moderate loads, but the 7403 grease had higher load capacity at 180 t. The choice must balance these factors.
b) Optimizing the guide surfaces of the screw gear can reduce over-constraint and extra resistance. Removing the upper sleeve reduced the resistance torque by about 29% at 180 t, and removing the copper sleeve reduced it by about 19%. Increasing the upper sleeve inner diameter had little effect.
c) Bearing corrosion has a major impact on screw gear resistance. At 180 t, corroded bearings increased the resistance torque by about 50%. Anti-corrosion measures are critical in humid and salt-spray environments.
d) The screw gear resistance increases with rotational speed. At 80 r/min, the resistance torque was 8% to 16% higher than at 40 r/min. The motor driving pressure difference reflects the internal resistance and can be used for condition monitoring.
e) After 60 transmission cycles, the screw gear resistance torque increased by about 16%, but it remained below the system driving capability. This provides a basis for estimating the service life of the support arm and transfer device.
f) Foreign matter and reducer lubrication failure should be controlled through design and process measures. They are indirect but important contributors to screw gear resistance and jamming.
My resistance characteristic analysis and experimental methods can provide methodological guidance for the design, testing, and maintenance of screw gear transmissions in launch pad systems. They can also support the development of heavy-lift launch pad screw gear mechanisms. The results show that by controlling lubrication, corrosion, over-constraint, and wear, the screw gear resistance can be kept within acceptable limits, ensuring reliable operation of the mobile launch pad.
In future work, I plan to extend the screw gear model to include thermal effects and dynamic load variations. I will also investigate advanced surface treatments for the nut and screw to reduce wear and extend service life. The experimental data and formulas presented here can serve as a baseline for comparing new screw gear designs and maintenance strategies.
Overall, my study demonstrates that the resistance properties of heavy-duty screw gear transmissions are governed by a combination of tribological, structural, and environmental factors. By quantifying each factor through controlled experiments, I have established a practical framework for improving the reliability and efficiency of screw gear systems in aerospace launch applications.