First, the following definitions attempt to clarify some common misconceptions surrounding this topic:
Backdriving: “A rotation of the input member caused by an external load (dynamic or static torque) being applied to the output.” With Spiroid and Helicon gearing technology the driving member is referred to as the ‘pinion’ while the driven member is referred to as the ‘gear’. Since normal operation involves powering the pinion to drive the gear, a backdriving scenario occurs when the face gear actively drives the pinion.
Self-Locking: “When the input fails to rotate after an external load applies a dynamic or static torque to the output.” In a self-locking scenario, the Spiroid or Helicon face gear is physically unable to drive the pinion.
Bi-Directional Operation: “The ability of the input to drive the output both clockwise and counter-clockwise.” Like most gear types, Spiroid and Helicon are able to operate bi-directionally. Prospective customers often confuse bi-directional operation (input driven) with the ability of a gear set to backdrive (output driven). Note: If an application requires single-direction operation, it is usually recommended that the gearset be designed in such a way that the load is driven by the low-side pressure angle. For clarity on pressure angles, see the image below.
Stick-Slip: “A spontaneous jerking motion that can occur while two objects are sliding over each other.” In a stick-slip environment, a self-locking gear set gives the impression of being backdriven via small, jerky movements. In reality, this movement is caused by a repeated separation and engagement between the pinion threads and the gear teeth. In effect, the pinion ‘runs away’ until the gear catches up to it in a momentary locked state. There is a fantastic old technical paper on this subject written by S. J. Mikina called “Worm-Drive ‘Jitters’ Can Be Avoided”. If you’re interested, let us know and we’ll forward a copy.
With all of the above in mind it holds true that Spiroid gearing technology is capable of being designed as either ‘backdriveable’ or ‘self-locking’. An important reminder, though: It is usually not advisable to rely exclusively on the self-locking characteristics of the gear set to serve as the system’s only brake. This is particularly true in applications with heavy vibrations which have a tendency to induce the ‘stick-slip’ jitters described above.
So what are the design parameters of backdriving and self-locking gear sets? When designing with Spiroid gear technology, we first pay close attention to pinion O.D., pinion thread lead, and the resultant pinion thread angle. The image below highlights the pinion’s thread angle. These are the initial indicators of whether or not a particular design will backdrive or self-lock. As a general rule of thumb – and with all else being equal – a thread angle of ≤5º will usually self-lock while a 10º+ thread angle should backdrive. As always, specific application details – i.e. surface finish, lubrication type, and vibration characteristics – will dictate whether or not these general guidelines hold true.
Since these are ‘rules of thumb’ that don’t always hold true, we will generally confirm things by calculating the theoretical backdriving efficiency. This is the final level of review when it comes to understanding whether a particular design will backdrive or self-lock. Typically, any backdriving efficiency less than -10% will be self-locking, while a backdriving efficiency of 10% or greater should backdrive. When backdriving efficiency falls between -10% and +10%, our engineers will alter the design to ensure that backdriving efficiency falls well beyond the preferred side of that range. Or, alternatively, they will insist upon thorough testing to validate.
What does all of this mean for Spiroid’s customers? It means that while it is often true that 15:1 reductions (and less) will backdrive and 25:1 (and greater) will self-lock, these reduction-based guidelines are not always true. If your application hinges on the ability to either backdrive or self-lock it is imperative that we make certain by taking a closer look at the geometry.
Many modern gear applications demand controllable ‘near zero’ backlash. For example, applications requiring precise positioning or indexing, or systems with frequent load reversals, benefit from simple backlash adjustment.
Both Helicon and Spiroid pinions have threads of constant lead and pressure angle. A Helicon pinion is completely insensitive in its axial position. Spiroid pinions are insensitive with the limits of movement necessary for backlash control.
The same is true for Helicon and Spiroid gears as far as their axial position is concerned. This feature provides for easy backlash adjustment by merely moving the gear along its axis.
Since the Spiroid pinion also is adjustable along its axis within a range of positions, there are new opportunities for finer backlash control. No other type of gear offers this feature in such a simple and direct way.
Before describing specific Helicon and Spiroid gear set characteristics, it should be noted that all gears can, at best, obtain only line contact under no load at any instantaneous position of mesh. Total length of line contact is one criterion of gear load carrying capabilities. Another is contact line movement during the engagement cycle. The contact line should not be stationary. It should sweep the entire available tooth surface. Movement or sweep of contact line brings freshly lubricated and cool areas into mesh. This applies to all gears. Other factors, such as relative curvature of contacting surface and inclination of the contact lines relative to sliding velocity, must also be considered.
In Helicon and Spiroid pinions the line of instantaneous contact is an almost radial line on each convolution of the pinion thread. It is almost perpendicular to the sliding velocity, and results in a full sweep of contact on the pinion and gear.
By contrast, the instantaneous line of contact on small worm gears is only slightly inclined to the direction of sliding velocity. This results in a narrow band of contact on the worm thread.
The efficiency of a gear set is a measure of the power lost in the gear mesh which turns into heat that must be dissipated. Thus, a gear set which is 70% efficient loses 30% of the power input and transmits 70%. The efficiency of Spiroid and Helicon gear sets, as with all other gear types, is a function of the gear set geometry, i.e. the pressure angles, and the coefficient of friction.
Helicon and Spiroid gear set geometry and ideal lubricating characteristics provide for inherently high efficiencies when compared to worm gears.
As with other gear types, the higher the gear set ratio, the lower the efficiency. Sometimes however, the low efficiency can be an asset, particularly when ‘self-locking’ is desired. Self-locking is the inability of the gear to drive the pinion, or back drive, and is a characteristic common to higher ratio gear sets.
Special design considerations can accommodate application requirements for higher dynamic efficiencies or self-locking efficiencies through the custom design versatility of Spiroid or Helicon gearing.
Gear accuracy is the most important factor in obtaining maximum quietness and minimum vibration. However, at high pitch line velocities even small inaccuracies can produce a pronounced noise, particularly if they occur at regular frequencies. Therefore, to achieve quiet gears, accuracy is combined with modifications in the profile and lead of the gear teeth, permitting them to engage gradually.
The contact lines on Helicon and Spiroid pinions are nearly radial, and sweep through the entire length of the teeth. At such great tooth length, very effective modifications can be made at the entering and leaving side, so that the pinion teeth “cam” into and out of action gently. The commonly used term for these modifications is “crowning”. Incorporated in either the pinion or the gear, and in conjunction with the large number of teeth in simultaneous engagement, it accounts for the fact that Helicon and Spiroid gears are inherently very quiet.
Many factors combine to give Helicon or Spiroid gears their superior strength and high surface durability characteristics. These factors are:
The number of teeth in simultaneous contact
The individual contact lines in relation to the sliding velocity
Large radii of curvature at the contact lines
Greater latitude in choice of materials
In situations where high surface durability, high static or running loads or shock loads are encountered, both members of the Spiroid gear set are made of hardened steel, the optimal choice among gear materials.
Indexing accuracy of gears is often given in minutes or seconds of arc. Other applications demand that the rotational speed of the gear not vary more than a fixed percentage of the average speed.
Like all gears, Helicon and Spiroid gears inherit their initial accuracy from the machines which produce them. However, Helicon and Spiroid gears have features which minimize the effect of machine errors and permit correction. For example, the large number of pinion teeth in simultaneous contact with gear teeth helps average out some of the errors existing on the individual teeth.
Gear tooth and pinion thread runouts cause angular velocity fluctuations the magnitude of which is affected by pressure angle. The greater the pressure angle, the greater the fluctuation in angular velocity. Therefore, the low pressure angle side should be used as the driving side in applications requiring low angular velocity fluctuations.
Helicon and Spiroid gears have many teeth in simultaneous contact. This, coupled with the fact that each pinion tooth contacts its mating gear tooth along a line perpendicular to the sliding velocity, leads to a number of important advantages.
The number of teeth in contact depends on the number of teeth in the gear member. Generally, 10% of the gear teeth are in simultaneous contact. However, even in the low ratio range, there are two to three times the number of teeth in contact than in worm gears. On higher ratios, there are many times more.
For any gear of a given diameter, a higher ratio means more teeth – which means finer pitch. Since contact between the Helicon or Spiroid pinion and gear extends over the entire length of the pinion, a finer pitch or shorter lead gives a proportionately increased number of teeth in simultaneous contact, and thus, very little sacrifice in capacity. With worm gears, for instance, the upper ratio limit is usually about 80:1. After selecting a pitch sufficient to carry the required load (bearing in mind that, at most, the worm set has two teeth in contact) the gear becomes too large. Therefore, in the case of worm gearing, higher ratios are usually handled by multiple reductions.
Helicon or Spiroid gears, however, do not have an upper limitation. For a given gear diameter, a higher ratio means a shorter lead, and finer pitch, but more teeth in contact. As a results, single reduction ratios of 400:1 are possible.