Screw Accuracy
Kerk uses a unique precision rolling process for screw manufacturing. Standard lead accuracy for Kerk screws is .0006 in./in. (mm/mm). Lead accuracies are available up to .0001 in./in. (mm/mm). Kerk can provide laser-interferometer generated mapping for our screws. Assemblies have an extremely high bi-directional repeatability of 50 micro-inches (1.25 micron).
End Machining
Kerk can custom machine screws to your specifications or provide cut-to-length screws for your own machining.
Critical Speed
This is the rotational speed at which a screw may experience vibration or other dynamic problems. See CRITICAL SPEED CHART to determine if application parameters result in speed approaching critical. To minimize critical speed problems: use a longer lead, choose a larger diameter or increase bearing mount support.
Lengths
Lengths can be specified up to 12 ft. (4M) from stock, (depending on diameter and lead). Cut to length screws are offered in 6" increments (6", 12, 18,....) +1.0"/-0".
Lead
Advancement per revolution. All screws are listed by lead, not pitch. Lead = Pitch x Number of Starts
Pitch
Crest-to-crest distance or one divided by threads per inch. (On a multiple start thread, the pitch equals the lead divided by the number of starts.)
Traverse Speed
The nut materials we use provide long wear-life over a wide variety of conditions. However, very high loads and/or speeds will accelerate nut wear. Special materials may be required for these situations. We offer the following guidelines for continuous duty linear traversing speeds for optimum life:
Lead |
Traverse Speed |
|
| 1/10" - ½" | 4 in./sec. | |
| 1/2" - 1" | 10 in./sec. | |
| 1" - 2 ½" | 30 in./sec. | |
| 1 - 12mm | 100mm/sec.. | |
| 12 - 25mm | 250mm/sec. | |
| 25 - 60mm | 60mm/sec. |
Maximum Load
Although Kerk Anti-Backlash Assemblies are capable of withstanding relatively high loads without catastrophic failure, these units have been designed to operate under the loading shown in the size charts.
Efficiency
Efficiency is the relationship of work input to work output. It should not be confused with mechanical advantage. Listed efficiencies are theoretical values based on Kerkote™ TFE coated screws.
Torque
The required motor torque to drive a lead screw assembly is the sum of three components: the inertial torque, drag torque, and torque to move load. It must be noted that this is the torque necessary to drive the lead screw assembly alone. Additional torque associated with driving frictional bearings and motor shafts, moving components, and drag due to general assembly misalignment must also be considered.
Inertial Torque: |
|||
| Tj= I α | Where | I = screw inertia | |
| α = angular acceleration |
Drag Torque
Kerk Anti-Backlash Assemblies are typically supplied with drag torque of 1 to 7 oz.-in. The magnitude of the drag torque is dependent on the standard factory settings or settings specified by the customer. Generally, the higher the preset force, the better the Anti-Backlash characteristics. Torque-To-Move Load: |
||
| LOAD x LEAD | ||
| TL= | ||
| 2 π x EFFICIENCY | ||
Screw Straightness
Screw straightness is measured as Total Indicator Runout(TIR). The standard Kerk straightness for lead screws is .003"/ft. Kerk can provide tighter specifications on customer request.All screws are hand straightened before shipping.
| SCREW INERTIA | ||
|---|---|---|
| Series | Screw Sizeinches (mm) |
Screw Inertia (oz.-in. sec.2/in.) |
| 4000 | 1/4" (6) |
.3 x 10-5 |
| 5000 | 5/16" (8) |
1.5 x 10-4 |
| 6000 | 3/8" (10) |
1.5 x 10-5 |
| 7000 | 7/16" (11) |
3.5 x 10-5 |
| 8000 | 1/2" (13) |
5.2 x 10-5 |
| 10000 | 5/8" (16) |
14.2 x 10-5 |
| 12000 | 3/4" (19) |
30.5 x 10-5 |
| 14000 | 7/8" (22) |
58.0 x 10-5 |
| 15000 | 15/16" (24) |
73.0 x 10-5 |
| ANTI-BACKLASH LIFE | ||
|---|---|---|
| Series | Without Kerkote™ TFE Coating inches (cm) |
With Kerkote™ TFE Coating inches (cm) |
| ZBA | 5 to 10 million (12 to 25 million) |
15 to 40 million (38 to 100) million |
| ZBX | 40 to 60 million (100 to 150 million) |
150 to 200 million (380 to 500) million |
| KHD | 80 to 100 million (200 to 250 million) |
180 to 230 million (450 to 580) million |
| NTB | 100 to 125 million (250 to 315 million) |
200 to 250 million (500 to 635) million |
| VHD | 200 to 225 million (500 to 570 million) |
300 to 350 million (760 to 880) million |
| B | N/A, Typical backlash .003 to .010 (.076 to .25) |
N/A, Typical backlash .003 to .010 (.076 to .25) |
| NTG | 5 to 10 million (12 to 25 million) |
15 to 40 million (38 to 100 million) |
Anti-backlash life is defined as the nut's ability to compensate for wear while maintaining its zero backlash properties. Above life data is based on 25% of the dynamic load rating. NTB style does not include mini series sizes. Life will vary with loading, operating environment, and duty cycle. The longer screw leads generally provide longer life.
| LEAD SCREW | |
|---|---|
| Material | Surface Finish |
| 303 Stainless Steel (options available) |
Better than 16 Micro Inch |
| NUTS | ||
|---|---|---|
| Material | Tensile Strength | Coefficient Of Expansion |
| Polyacetal with Lubricating Additive | 9,700 psi | 6.0 x 10-5in/in./ºF |
| Assembly | |
|---|---|
| Standard Operating Temp. Range |
Coefficient of Friction Nut to Screw |
| 32-200º F*(0-93ºC)* |
Static = .08 .08**Dynamic = .15 .09** |
* Very high or low temperatures may cause significant changes in the nut fit or drag torque. Please call Kerk for optional temperature range materials.
** with Kerkote™ TFE Coating
| Inch | Metric | ||
|---|---|---|---|
| .X | ± .02 | < L 4 | ± 0.1 |
| .XX | ± .010 | 4 < L =16 | ± 0.15 |
| .XXX | ± .005 | 16 < L =63 63 < L =250 |
± 0.2 ± 0.3 |