The choice between different types of lead screws and lead screw nuts available is generally made after taking into account the following factors.

**Working environment**

For work environments where there are no particular corrosive or oxidising agents our steel (L1320 and L1321) lead screws can be used.

Where these conditions are not met, we recommend using our stainless steel screws (L1322 and L1323) which are particularly suitable in the following cases:

- With a relative humidity of 70-80% and above.
- Immersed in water (including sea water).
- In the presence of particular corrosive agents such as chlorides. In case of highly corrosive agents please contact our Technical Department.
- In the food industry or pharmaceutical industry, where they are used with stainless steel or bronze nuts.
- Where the lead screws cannot be reached for lubrication. In particular, for lubricating “maintenance free” fi ttings they are coupled with plastic nuts.
- Where working temperatures are relatively high (above 200°C) - because stainless steel has a structure that is more suited to higher temperatures.

**Backdrive**

Irreversibility defi nes how much the nut can “backdrive” down the lead screw. If a nut cannot backdrive down the screw then it is irreversible. This is especially important if the lead screw and nut are being used in a vertical application i.e. in this situation no backdrive is normally acceptable.

Lead screws with a lead angle of <2° 30’ are completely irreversible i.e., they cannot backdrive. Lead screws with a lead angle >5° but <6° still have a good degree of irreversibility and may in some instances exhibit some backdrive.

Lead screws with a lead angle >6° have zero irreversibility, therefore the nut may backdrive down the lead screw with little or no load applied.

This is important to know in vertical applications.

**Working environment**

Our bronze and stainless steel 303 lead screw nuts, are resistant to standard oxidizing agents that occur in various applications.

Where corrosive agents are present, please contact our Technical Department for advice.

**In applications where the presence of added lubricant (grease or oil) is not allowed we recommend the use of self-lubricating plastic nuts.**

**The use of plastics can however be limited by the specific working conditions, therefore please consult our Technical Department should you wish. This is because plastics have excellent self-lubrication features, but at the same time have restrictions on the working temperature or moisture absorption problems, (as well as some mechanical properties that may not be suitable for the intended use).**

**Pitch**

The axial distance between threads. Pitch is equal to the lead in a single start screw.

**Lead**

The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts.

**Lead = Pitch x No. of starts**

For example: A 10mm diameter lead screw has a pitch of 2mm. On a single start lead screw the lead is also 2mm. On a twin start lead screw the lead is 4mm

**Screw starts**

The number of independent threads on the screw shaft, example one or two.

Automotion Components lead screws are produced with controlled straightness. Screw straightness is checked by measuring the variation of the deflection ƒ when the screw is supported at the ends on two fixed points and slightly rotated.

For example, the screw L1320.R30-06 has a straightness of 0.2 mm over 300 mm. This means that a screw 30x6 300 mm long resting on two fi xed points at the ends and rotated slightly displays a camber variation Δƒ less than 0.2 mm at all points on the screw.

**Straightness**

Δ ƒ = lead screw weight camber.

Good screw straightness gives operation with load always centred on the axis, hence uniform distribution of surface contact pressure between screw and nut. This allows smooth running and a regular rotation.

To select the correct size of lead screw and nut to use, please take into consideration the following three points:

- The size required to minimise the wear of the nut due to friction.
- The maximum load the screw can take before it bends.
- The maximum rpm the screw can run at before it begins to vibrate.

**Finding a size to minimise the wear of a nut**

Due to friction between the lead screw and nut, some of the power put into the lead screw is lost as heat. The solution is to limit the contact surface area between the lead screw and nut as this will help reduce the amount of friction and wear on the nut.

Lead screws are used to convert rotary motion into linear motion.

The efficiency of a lead screw and nut is defi ned as the amount of power you get from the nut in relation to the amount of power you put into the lead screw to begin with. For example, Pt (the power you put into the screw) divided by Pu (the power you get from the nut) = the effi ciency.

The efficiency depends on the friction between the contact surfaces of the lead screw and nut, and the lead angle of the thread.

The speed the nut moves along the lead screw must also be taken into consideration in order to limit the amount of friction in the system.

Please see the formulae on the next page which help in calculating the speeds in your application.

The contact surface pressure p is calculated using the following formula.

The sliding speed is the result of the speed at which the nut moves and the friction. Calculating the sliding speed helps you to calculate the required rpm of a screw if you know what speed the nut must move at, or help to calculate how fast the nut will move if you know the rpm of the screw.

You must first find the sliding speed of your lead screw and nut using one of the below formulae and then use this to calculate either rpm or output speed of the nut.

The sliding speed is calculated using one of the following formulae.

The critical rpm is the speed at which screw vibration begins to appear. This rpm speed must never be reached because the vibrations cause serious operating issues. The critical rpm depends on lead screw diameter, how the ends of the lead screw are supported, the free length (lg), and how accurately the lead screw is assembled.

For values shown in the following graphs assume a minimum safety factor for assembly accuracy as per the following chart.

Assembly accuracy | Conditions | Safety coefficient |

Good assembly accuracy: Nut alignment to screw within 0,05mm | Assembly to which the bearing mounts are assembled CNC machined very accurately. | 1,3 – 1,6 |

Average assembly accuracy: Nut alignment to screw within 0,10mm | Bearing mounts installed onto assembly that has not been CNC machined, alignments accurately checked after mounting. | 1,7 – 2,5 |

Low assembly accuracy:Nut alignment to screw within 0,25mm | Bearing mounts installed onto assembly that has not been CNC machined and alignment is not checked accurately. | 2,6 – 4,5 |

To find the critical rpm of a lead screw 40 mm diameter with 7 mm lead, 3 metres long with end support configuration double bearing one end, single bearing the other end (see graph 3) with average assembly accuracy.

Critical rpm graph 3 gives the critical rotation speed of 1000 rpm.

From the assembly accuracy coeffi cient chart we take the maximum value for the safety coefficient = 2,5

We can calculate the acceptable working speed at a maximum rpm of N_{max} = 1000/2,5 = 400 rpm

Supported one end only by double bearing

Supported each end by single bearings

Supported each end. Double bearing one end, single bearing the other end

Supported by double bearings each end

**Efficiency**

The numerical efficiency values of each limit are shown in the table ‘Trapezoidal Lead Screw and Nut Specifi cations’.

The larger the lead angle of a lead screw, the greater the efficiency is for the lead screw. It is therefore recommended, where possible, to use a lead screw with a lead angle as high as possible.

The lead angle for each size of screw can also be found in the trapezoidal lead screw and nut specifications table. To help reduce friction as far as possible, we make precision rolled trapezoidal lead screws with minimal roughness on the side of the threads, always less than 1μ Ra (usually 0.2 to 0.7 μ).

For applications where low friction is important, we also make wear resistant self lubricating flanged plastic nuts. The friction factor of these is 0.1 with the initial breakaway friction factor being 0.15.

Diameterx lead | Lead angle | Max. efficiency η ƒ=0.1* | Min. efficiency η ƒ=0.2* | Moment of inertia mm ^{4} | Radial play between screw and nut min. | Radial play between screw and nut max. | Axial play between screw and nut min. | Axial play between screw and nut max. |

10 x 2 | 4°02’ | 0,41 | 0,26 | 131 | 0,071 | 0,511 | 0,019 | 0,137 |

10 x 4 | 8°03’ | 0,58 | 0,40 | 131 | 0,071 | 0,511 | 0,019 | 0,137 |

12 x 3 | 5°12’ | 0,47 | 0,31 | 215 | 0,085 | 0,609 | 0,023 | 0,163 |

12 x 6 | 10°19’ | 0,63 | 0,46 | 215 | 0,085 | 0,609 | 0,023 | 0,163 |

14 x 3 | 4°22’ | 0,43 | 0,27 | 518 | 0,085 | 0,609 | 0,023 | 0,163 |

14 x 6 | 8°41’ | 0,59 | 0,42 | 518 | 0,085 | 0,609 | 0,023 | 0,163 |

16 x 4 | 5°12’ | 0,47 | 0,31 | 738 | 0,095 | 0,715 | 0,025 | 0,192 |

16 x 8 | 10°19’ | 0,63 | 0,46 | 738 | 0,095 | 0,715 | 0,025 | 0,192 |

18 x 4 | 4°33’ | 0,44 | 0,28 | 1434 | 0,095 | 0,715 | 0,025 | 0,192 |

18 x 8 | 9°02’ | 0,60 | 0,43 | 1434 | 0,095 | 0,715 | 0,025 | 0,192 |

20 x 4 | 4°03’ | 0,41 | 0,26 | 2534 | 0,095 | 0,715 | 0,025 | 0,192 |

20 x 8 | 8°03’ | 0,58 | 0,40 | 2534 | 0,095 | 0,715 | 0,025 | 0,192 |

22 x 5 | 4°40’ | 0,45 | 0,28 | 3232 | 0,106 | 0,761 | 0,028 | 0,204 |

22 x 10 | 9°16’ | 0,61 | 0,43 | 3232 | 0,106 | 0,761 | 0,028 | 0,204 |

24 x 5 | 4°14’ | 0,42 | 0,27 | 5175 | 0,106 | 0,806 | 0,028 | 0,216 |

24 x 10 | 8°25’ | 0,59 | 0,41 | 5175 | 0,106 | 0,806 | 0,028 | 0,216 |

26 x 5 | 3°52’ | 0,40 | 0,25 | 7884 | 0,106 | 0,806 | 0,028 | 0,216 |

26 x 10 | 7°42’ | 0,57 | 0,39 | 7884 | 0,106 | 0,806 | 0,028 | 0,216 |

28 x 5 | 3°34’ | 0,38 | 0,23 | 11539 | 0,106 | 0,806 | 0,028 | 0,216 |

28 x 10 | 7°07’ | 0,55 | 0,37 | 11539 | 0,106 | 0,806 | 0,028 | 0,216 |

30 x 6 | 4°03’ | 0,41 | 0,26 | 13650 | 0,118 | 0,903 | 0,032 | 0,242 |

30 x 12 | 8°03’ | 0,58 | 0,40 | 13650 | 0,118 | 0,903 | 0,032 | 0,242 |

32 x 6 | 3°46’ | 0,39 | 0,24 | 17580 | 0,118 | 0,903 | 0,032 | 0,242 |

32 x 12 | 7°30’ | 0,56 | 0,38 | 17580 | 0,118 | 0,903 | 0,032 | 0,242 |

36 x 6 | 3°19’ | 0,36 | 0,22 | 34540 | 0,118 | 0,903 | 0,032 | 0,242 |

36 x 12 | 6°36’ | 0,53 | 0,36 | 34540 | 0,118 | 0,903 | 0,032 | 0,242 |

40 x 7 | 3°30’ | 0,38 | 0,23 | 51030 | 0,125 | 0,955 | 0,033 | 0,256 |

40 x 14 | 6°58’ | 0,54 | 0,37 | 51030 | 0,125 | 0,955 | 0,033 | 0,256 |

44 x 7 | 3°09’ | 0,35 | 0,21 | 81820 | 0,125 | 0,955 | 0,033 | 0,256 |

50 x 8 | 3°10’ | 0,35 | 0,21 | 136900 | 0,132 | 1,062 | 0,035 | 0,285 |

55 x 9 | 3°15’ | 0,36 | 0,22 | 189550 | 0,140 | 1,125 | 0,038 | 0,301 |

60 x 9 | 2°57’ | 0,34 | 0,20 | 302600 | 0,140 | 1,125 | 0,038 | 0,301 |

70 x 10 | 2°48’ | 0,33 | 0,19 | 587500 | 0,150 | 1,135 | 0,040 | 0,304 |

80 x 10 | 2°26’ | 0,30 | 0,17 | 1069000 | 0,150 | 1,135 | 0,040 | 0,304 |

90 x 12 | 2°36’ | 0,31 | 0,18 | 1658000 | 0,170 | 1,295 | 0,046 | 0,347 |

95 x 16 | 3°21’ | 0,37 | 0,22 | 1647000 | 0,190 | 1,500 | 0,051 | 0,402 |

100 x 16 | 3°10’ | 0,35 | 0,21 | 2124000 | 0,190 | 1,500 | 0,051 | 0,402 |

120 x 16 | 2°36’ | 0,31 | 0,16 | 5130000 | 0,190 | 1,500 | 0,051 | 0,402 |

Diameter x lead | Lead accuracy μ/300mm | Straightness mm/μ |

10 x 2 to 20 x 4 | 0,1 | 0,5/300 |

22 x 5 to 60 x 9 | 0,1 | 0,2/300 |

70 x 10 to 80 x 10 | 0,1 | 0,4/300 |

90 x 12 | 0,2 | 0,5/300 |

95 x 16 to 120 x 16 | 0,2 | 1,0/300 |

h_{1} = 0,5 P

h3 = h4 = h_{1} + x = 0,5 P + x

z = 0,25 P = h_{1}/2

d_{3} = d - 2 h_{3}

d_{2} = D_{2} = d - 2 z = d - 0,5 P

D_{2} = d + 2 x

P = thread pitch

d_{1} = nominal thread diameter

d_{3} = core thread diameter

x = bottom play

y = top deviation for screw

s = 0,26795 y

r_{1} max. = 0,5 x

r_{2} max. = x

**Reversibility/backdrive**

No backdrive where lead angle <2° 30’.

At angles up to 5-6° there is a low potential of system backdrive.

Backdrive is important in vertical applications.

The load acting upon a nut that would tend to compress or buckle the screw. Also referred to as column loading, this rating is effected by the load, support type, screw diameter, and length between the load point and support housing.

Normally, a screw also experiences a tension load (a force which attempts to stretch the screw). For vertical applications, it is better to configure the screw assembly so that the screw is in tension, and not in compression. To find maximum compression load a screw can take, please refer to critical axial load technical pages.

The load acting upon a nut that would tend to stretch the screw. The maximum tension load of a screw assembly is the load rating of the nut. For vertical applications, it is better to confi gure the ball screw assembly so that the screw is in tension and not in compression.

When choosing a lead screw you need to consider the critical axial load to avoid the screw bending under excessive loads. This is the buckling load. This is important where the end screws are being used in compression. The critical axial load depends on the core diameter of the lead screw (d_{з}), how the lead screw is supported at each end, and the free length of screw (le). In the graphs below, please allow a minimum safety factor of > 2.

Supported each end by double bearings

**Example**

Find the allowable axial load of a 30x6 screw 3000mm long with constraint conditions as in drawing 1. From the accompanying graph take Fmax=11kN with safety factor of 2 and assume Fallw =11/2=5,5kN.

Supported each end by single bearings

Supported each end. Double bearings one end, single bearing the other end

Supported one end only by double bearing

The torque necessary to move a screw and nut system is calculated from the following equation.

C = torque (input) (Nm)

F = axial force on nut (N)

P = true lead of screw (mm)

η = efficiency (assume effi ciency with first breakaway friction factor f = 0.2

**Example**

Find the torque required to move a 30x6 lead screw and nut

Resistant axial force = 10.000 N

η = 0.26

Screw lead = 6 mm

The torque value however does not consider the efficiency of mechanical parts moving together with the screw system, such as bearings, belts or other transmission components. In a planning project phase, an increase between the 20% and 30% of the theoretical value is recommended. If electric motors with low static torque are used assume another increase of 50% to find nominal torque.

**C = 36.7 (Nm) • 1.3 • 1.5 = 71.6 Nm**

The power necessary to move a trapezoidal screw and nut system is calculated from the following equation.

9550 is a constant

P = power (kW)

C = torque (Nm)

n = rpm

Calculate the power necessary to move the screw 30 x 6 in the above example at 600 rpm

This is the minimum power necessary to move the system

In all three situations described, the wear of the nut is affected by the lubrication used during operation and as such, giving accurate figures for the life expectancy of the nut is impossible.

Extra care must be taken when the temperature of the application is above +140°/150°C as such temperatures can damage lubricants and as a consequence, cause the nut to wear quicker. In these situations we recommend the use of lubricants designed for high temperatures.

During the selection process we must also check that the inertia forces present during acceleration and deceleration are relatively low so that the value of p●Vst remains within the controlled limits. Whereas this calculation is difficult, in the presence of a non-uniform movement or under great variations, safety factors reported in the chart below must be considered.

Load type | f_{i} |

Loads with constant acc. / dec. controlled | from 1,00 to 0,50 |

Loads with constant start and stop at tear | from 0,50 to 0,33 |

Loads and speed greatly variable | from 0,33 to 0,25 |

Loads in presence of shocks and vibrations | from 0,25 to 0,17 |

The coefficient f_{i} is used to correct the value of (p●Vst) max derived from the ‘Sliding Condition for Bronze’ graph, considering the maximum allowable sliding speed in relation to the contact surface in working conditions. Working area limits (A, B or C) must be taken into consideration.

To calculate the admissible p●Vst of the nut in working conditions the following must be used

**p●Vst am = (p●Vst) _{max}●ƒ_{i}**

Selecting a bronze nut which must operate continuously and remain within the maximum limit value of p●Vst = 21 (Area A), with good lubrication. Constant axial load without relevant variations, with forces of inertia limited by controlled acceleration/deceleration.

Axial Load - F = 1200N (1Kg ƒ=9,81N)

Constant motion speed - Vtr = 2,8m/min

Evaluation of p●Vst using nut L1331.R30-06

(bronze flanged nut with thread Tr 30x6 1 start, right)

F = Axial Force (N)

At = Contact Surface Area (mm^{2}).

For standard nuts each At value is listed in the product tables.

When using a bronze nut, calculate the sliding speed and use the graph below to see if it is suitable. The graph has three areas each characterised by certain working conditions. These figures are evaluations obtained from the results of experiments we have carried out. Good lubrication is always required, if little or no lubrication is used the working conditions may vary greatly.

**Sliding condition for bronze.**

**Area A** Area A is enclosed by the limit p•Vst = 21 (N/mm2•m/min)

These are the best operating conditions.

Continuous operation is possible as the amount of friction produced within these limits p•Vst is pretty low. Therefore the life of the nut is very good.

**Area B** Area B is enclosed by the limit p•Vst = 80 (N/mm2 • m/min)

These operating conditions are more severe. Constant lubrication is required to help prevent wear of the nut and improve its lifetime.

Continuous operation is possible for limited periods only as the amount of friction produces overheating of the nut. Although lubrication helps reduce heat, the life of the nut is limited.

**Area C** Area C is enclosed by the limit p • Vst = 250 (N/mm2 • m/min)

In this area, the operating conditions are very severe.

Continuous operation is not possible.

Even with good lubrication the amount of friction and heat produced causes rapid wear of the nut.

The sliding speed is calculated using formulae:

In order to remain within the continuous working conditions, corrected by the safety factor fi from the table, in this case =0,77, the maximum allowable value of p●Vst is:

As the maximum allowable value of p●Vst is lower than the value obtained with a nut L1331.R30-06, we shall try using a nut L1335.R36-06 (square bronze nut with 36x6 thread)

The contact surface pressure is:

The value obtained is now lower than the allowable one, therefore the L1335.R36-06 will be suitable.

In applications where low noise is important or where lubrication is not allowed (grease or oil), self lubricating plastic nuts are recommended. The use of plastics is very constrained by the actual working conditions, we suggest discussing the application with our technical department and not relying on a choice based only on intuition. This is because plastic materials have good features such as low friction and self-lubrication, but at the same time limitations caused by operating temperatures, hygroscopic problems, or certain mechanical features that may not be suitable for the intended use. An advanced study of the application in this case is therefore required in order to obtain optimum performance.

Regarding the plastic nuts, the study of the product p●Vst allows you to draw a chart which shows a curve that limits the values of p●Vst within which we have a gentle flow of the surfaces in contact with limited wearing of the nut and constant in time. Operating outside the limit drawn on the chart is not possible as in this case as the nut would wear quickly.

The graph below shows the limit of p●Vst of the cylindrical nut L1343. As this plastic is resistant to wear but not self-lubricating, the following limits have been shown when the nut is dry, and when it is lubricated intermittently.

**Sliding condition for nuts L1343**

Test conditions:

• Continuous operation.

• Temperature 23°C.

• Relative humidity approx 50%.

The graph below shows the limit of p●Vst of nut L1342. The plastic used for the L1342 features a strong resistance to wear and complete self-lubricating properties.

**Sliding conditions for self-lubricating plastic nuts L1342**

Test conditions:

• Continuous operation.

• Temperature 23°C.

• Relative humidity approx 50% with no lubrication.

During the selection process check that the inertia forces present during acceleration and deceleration are relatively low so that the value of p●Vst remains within the required controlled limits. Whereas this calculation is difficult, in the presence of a non-uniform movement or under great variations a safety factor in the chart below must be applied.

Load type | f_{i} |

Loads with constant ramps of acc. / dec. controlled | from 1,00 to 0,50 |

Loads with constant start and stops | from 0,50 to 0,33 |

Loads and speed greatly variable | from 0,33 to 0,25 |

Loads in presence of shocks and vibrations | from 0,25 to 0,17 |

Using plastic nuts L1343 or L1342, the value of p●Vst must be corrected in relation to the working temperature. Plastic becomes softer at higher temperature and can handle less load. At lower temperatures, it becomes harder and takes heavier loads. Correction factor f_{t} is shown in the graph below.

Plastic nuts operating in on and off cycles for relatively short periods of time do not reach the limit of the maximum permissible temperature of the surface in contact with the screw. This temperature is a constraint that mainly contributes to limit the values of p●Vst in graphs for nuts L1343 and L1342 in continuous operation. The value of p●Vst admissible when operating in on and off cycles is higher than the value in continuous cycles.

Using the graph below find the value of factor f_{c}. The curves of x represent the relationship between the downtime and the working time of the nut.

- 1 x downtime same as working time.
- 2 x downtime twice that of the working time.
- 3 x downtime three times the working time.
- 4 x downtime four times the working time.

The values of the three coefficients f_{i}, f_{t}, f_{c} are used to correct the value of the product (p●Vst) max read from graph on page 235 (for nut L1343) or the graph on page 236 (for nut L1342), considering the maximum admissible sliding speed in test conditions in relation to the contact surface pressure in working conditions.

To calculate the allowable p●Vst of the nut in working conditions

**p●Vst am = (p●Vst) _{max}●ƒ_{i} ●ƒ_{t} ●ƒ_{c}**

Selecting the correct size of L1342 flanged self-lubricating plastic nut which operates in the following conditions:

- Constant axial load with forces of inertia limited by controlled acceleration/deceleration of F = 1750 N.
- Moving speed = 10 m/min.
- Working time = 20 seconds, with downtime = 60 seconds.
- Working environment temperature = 50°C.
- No lubricant.

The nut L1342 is perfectly self-lubricating and therefore suitable to use in this application so we select a size we think may be suitable. Then we calculate the product p●Vst to see if it is lower than the admissible value of p●Vst as per the graph on page 235, then correct it with the coefficients f_{i}, f_{t} and f_{c} on pages 236 and 237.

We choose the L1342.R40-07, and calculate the contact surface pressure:

F = Axial Force (N)

At = Contact Surface Area(mm^{2}). For standard nuts each At value is listed in the product tables

The sliding speed is calculated:

The value of the product p●Vst is

**p●Vst = 0,25 N/mm ^{2} ● 164 m/min ≅ 41 N/mm^{2} ● m/min**

Now we calculate the admissible value of p•Vst in working conditions. From page 235 we see that in continuous operation at 23°C with p = 0,25 (N/mm^{2}) the admissible value of Vst is @ 140(m/min)

i.e.** (p●Vst) _{max} = 0,25 ● 140 = 35 N/mm^{2} ● m/min**

From graph on page 236 we read the value of coefficient f_{i}. In our case f_{i} may be 0,75

From graph on page 236 we read the value of coefficient f_{t}. In our case, in the working environment temperature of 50°C we may assume f_{t} may be 0,8

From graph on page 237 we read the value of coefficient f_{c}. In our case with working time of 20 seconds and downtime of 60 seconds therefore

We assume f_{c} = 3,7

The maximum admissible value of p●Vst in our case:

**p●Vst am = (p●Vst) _{max} ● f_{i} ● f_{t} ● f_{c} = 35 N/mm^{2} ● m/min**

**● 0,75 ● 0,8 ● 3,7 = 77,7 N/mm ^{2} ● m/min**

As the value of the product p●Vst is in this case lower than the admissible value, the nut L1342.R40-07 may be used in this application

Using the experimental values it is possible to give an indication of the lifetime a plastic nut may have. The parameters that aff ect the life of a plastic nut are as follows:

- Value of the contact surface pressure p (N/mm
^{2}). - Value of the sliding speed Vst (m/min).
- Correction factor f
_{c}of the on and off cycle. - Constant of the resistance to the wear of the plastic in exam derived from experimental tests k

All data shown below is for combining our plastic nuts with our precision rolled screws as we guarantee a surface roughness less than 1 μm Ra.

Using plastic nuts for a lathe screw application is not suitable.

The following calculations and considerations are for screws working at a temperature of approx 20/25°C with relative humidity from 30% to 70%. For working conditions with a different temperature and humidity, you should contact our technical department.

To calculate the lifetime we use the following formula

Value of the constant k for plastic nuts

L1343 k = 10,5●10^{-5}

L1342 k = 2,5●10^{-5}

Selecting the size and calculating the lifetime of nut L1342 operating in the following conditions:

- Constant axial load forces of inertia limited by controlled ramps of acceleration/deceleration of F = 450 N.
- Motion speed = 10 m/min.
- Working time = 12 seconds, with downtime = 12 seconds.
- Distance covered in 12 seconds at 10 m/min ≅ 2000 mm.
- Working environment temperature ≅ 22°C.
- Working environment relative humidity ≅ 40%:60%.
- No lubrication.
- Minimum lifetime requested: the coupling screw/nut must work for 200,000 cycles (i.e. approx 1,330 hrs at the above conditions) increasing the axial play in respect of the initial value of 0,1 mm.

Nuts L1342 are perfectly self-lubricant and therefore suitable to work in these conditions.

As the motion speed requested (10 m/min) we consider using L1342.R28-10.

To verify the p●Vst see example on page 238

Contact surface pressure is calculated:

The sliding speed is calculated:

The value p●Vst is:

**p●Vst = 0,125 N/mm ^{2}● 80,7 m/min ≅ 10 N/mm^{2}●m/min**

Now we calculate the admissible p●Vst at the considered conditions.

From graph on page 235 we see that in continuous working conditions at 23°C with p = 0,125 (N/mm2) the admissible value of Vst is ≅ 180 (m/min)

**i.e. (p●Vst) _{max} = 0,125 ● 180 = 22,5 N/mm^{2}●m/min**

From chart on page 236 f_{i} = 0,75

From graph on page 236 f_{t} = 1,00

From graph on page 237 f_{c} = 3,00

**p●Vst am = p ● Vst ● f _{i} ● f_{t} ● f_{c} = 22,5 N/mm^{2}●m/min**

**● 0,75 ● 1 ● 2 = 33,75 N/mm ^{2}●m/min**

As the value of p●Vst is here less than the allowable one, the L1342.R28-10 may be used in this application.

Now we calculate the wear of the nut in continuous working conditions and therefore an increase of the axial play of 0,2 mm using graph/table

Therefore 800 working hours, at the speed of 10 m/min, correspond to the following distance

We have a lifetime of 1,600 hours.

**Advantages**

Easy to install. Allows for some misalignment at installation. Compact system, small footprint compared to other rail systems. Preload adjustable by hand.

**Disadvantages**

Although it can take very large loads it cannot take anywhere near as much load as the linear guideways rail system.

**Advantages**

Cheaper alternative to the compact rail. Use of T and U rails allows for misalignment at installation. Preload adjustable by hand.

Available in AISI 316L stainless steel suitable for use in applications requiring a high level of corrosion resistance, including sea water.

**Disadvantages**

Cannot take as much load as other systems. Not suitable for moment loads.

**Advantages**

Can take extremely high loads including moment loads. Very smooth in operation.

**Disadvantages**

Must be aligned very accurately which costs time and money preparing the mounting surface properly.

**Advantages**

Available in a range of materials and sizes. A length of shaft bar is typically cheaper than the cost of an equivalent size precision linear rail.

**Disadvantages**

A larger diameter shaft would be required when there are long lengths and high loads involved compared with the size of an equivalent shaft support rail you would need. This is because they would only be supported at the ends and the shaft ends could bend in the middle if the diameter was too small.

**Advantages**

Shaft support rail systems have a shaft support along the full length so are less likely to flex.

**Disadvantages**

Similar to linear guideways, must be aligned very accurately otherwise any mis-alignment may cause the system to ‘snatch’.

Alexia House

Little Mead Industrial Estate

Cranleigh

Surrey, GU6 8NE (UK)

Telephone 0333 207 4498

or + 44 (0) 1483 266 774

Fax + 44 (0) 1483 266 775

Email sales@automotioncomponents.co.uk

Company registration no.2761902

Company VAT number GB 566990288

For other bank details visit our FAQs page.

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We are entering the next phase of our relocation to a larger factory in Chichester, West Sussex. The next few weeks will see products start to be moved from our current site in Cranleigh, Surrey to the new warehouse.

There are thousands of parts to be relocated and whilst we are doing our best to ensure a smooth transition, this may cause slight delays to some shipments.

We expect to be have all parts relocated and full production capability for our new 35,000ft² facility running by mid-March. Thank you for your understanding.

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