WO2008093178A2 - An improved friction screw mechanism - Google Patents

An improved friction screw mechanism Download PDF

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Publication number
WO2008093178A2
WO2008093178A2 PCT/IB2007/055284 IB2007055284W WO2008093178A2 WO 2008093178 A2 WO2008093178 A2 WO 2008093178A2 IB 2007055284 W IB2007055284 W IB 2007055284W WO 2008093178 A2 WO2008093178 A2 WO 2008093178A2
Authority
WO
WIPO (PCT)
Prior art keywords
screw
flanks
axially
nut
screw mechanism
Prior art date
Application number
PCT/IB2007/055284
Other languages
French (fr)
Other versions
WO2008093178A3 (en
Inventor
Emmanuel Eyraud
Daniele Volpiano
Original Assignee
Aktiebolaget Skf
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Aktiebolaget Skf filed Critical Aktiebolaget Skf
Publication of WO2008093178A2 publication Critical patent/WO2008093178A2/en
Publication of WO2008093178A3 publication Critical patent/WO2008093178A3/en

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60T—VEHICLE BRAKE CONTROL SYSTEMS OR PARTS THEREOF; BRAKE CONTROL SYSTEMS OR PARTS THEREOF, IN GENERAL; ARRANGEMENT OF BRAKING ELEMENTS ON VEHICLES IN GENERAL; PORTABLE DEVICES FOR PREVENTING UNWANTED MOVEMENT OF VEHICLES; VEHICLE MODIFICATIONS TO FACILITATE COOLING OF BRAKES
    • B60T17/00—Component parts, details, or accessories of power brake systems not covered by groups B60T8/00, B60T13/00 or B60T15/00, or presenting other characteristic features
    • B60T17/18—Safety devices; Monitoring
    • B60T17/22—Devices for monitoring or checking brake systems; Signal devices
    • B60T17/221—Procedure or apparatus for checking or keeping in a correct functioning condition of brake systems
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60T—VEHICLE BRAKE CONTROL SYSTEMS OR PARTS THEREOF; BRAKE CONTROL SYSTEMS OR PARTS THEREOF, IN GENERAL; ARRANGEMENT OF BRAKING ELEMENTS ON VEHICLES IN GENERAL; PORTABLE DEVICES FOR PREVENTING UNWANTED MOVEMENT OF VEHICLES; VEHICLE MODIFICATIONS TO FACILITATE COOLING OF BRAKES
    • B60T11/00—Transmitting braking action from initiating means to ultimate brake actuator without power assistance or drive or where such assistance or drive is irrelevant
    • B60T11/04—Transmitting braking action from initiating means to ultimate brake actuator without power assistance or drive or where such assistance or drive is irrelevant transmitting mechanically
    • B60T11/046—Using cables
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60T—VEHICLE BRAKE CONTROL SYSTEMS OR PARTS THEREOF; BRAKE CONTROL SYSTEMS OR PARTS THEREOF, IN GENERAL; ARRANGEMENT OF BRAKING ELEMENTS ON VEHICLES IN GENERAL; PORTABLE DEVICES FOR PREVENTING UNWANTED MOVEMENT OF VEHICLES; VEHICLE MODIFICATIONS TO FACILITATE COOLING OF BRAKES
    • B60T13/00—Transmitting braking action from initiating means to ultimate brake actuator with power assistance or drive; Brake systems incorporating such transmitting means, e.g. air-pressure brake systems
    • B60T13/02—Transmitting braking action from initiating means to ultimate brake actuator with power assistance or drive; Brake systems incorporating such transmitting means, e.g. air-pressure brake systems with mechanical assistance or drive

Definitions

  • the present invention relates to an improved friction screw mechanism for a linear electromechanical actuator, in particular for tensioning and releasing a cable for operating a parking brake.
  • Braking devices associated with linear electromechanical actuators comprise an electric motor and a screw mechanism which converts the rotary movement imparted to the rotor of the electric motor into a linear movement of an output member which applies a braking force to the parking brakes of the vehicle by means of a Bowden cable.
  • the friction screw mechanisms exert a very high traction force in only one of the two possible directions of displacement and therefore it is only one of the two flanks of the threads on the screw and on the screw nut which works with sliding friction under a high load.
  • the object of the present invention is to provide an improved screw mechanism in order to overcome the abovementioned drawback.
  • the screw mechanism is associated with a spring member which, when the mechanism is in a condition where it does not produce a traction force for operating the parking brake, is able to impart an axial thrust to the screw or the screw nut such as to displace the screw and the screw nut relative to each other so as to move away from each other those facing flanks of the threads of these two members which co-operate with sliding friction when the mechanism produces the abovementioned traction force.
  • Figure 1 is a cross-sectional perspective view of an electromechanical actuator which includes a first embodiment of a screw mechanism according to the invention
  • Figure 2 is an enlarged axially sectioned view of a part of the screw mechanism according to Figure 1;
  • Figure 3 is a cross-sectional view of the part shown in Figure 2 in a different operating position
  • Figure 4 is an axially sectioned view of a part of a second embodiment of a screw mechanism according to the invention.
  • a linear electromechanical actuator intended to operate the parking brake (not shown) of a motor vehicle.
  • the actuator 10 is associated with an electric motor (not shown) which, by means of a gear reduction unit 11 and an associated screw mechanism 20, imparts linear pulling and release movements to a Bowden cable (not shown) connected to the parking brake.
  • the screw mechanism 20 comprises a screw nut 21 rotatable about a longitudinal axis x and a threaded rod or screw 22 displaceable along the axis x.
  • the screw nut 21 is rotatably supported inside a rigid casing 30 by a bearing 31 and receives the rotational movement from a gear 12 of the reduction unit 11.
  • the screw mechanism converts the rotational movement of the screw nut 21 into a linear translation movement of the screw 22.
  • the screw 22 has an output end 23, which can be connected to a Bowden cable (not shown), and an opposite end 24 which has a transverse surface 25.
  • the transverse surface 25 forms part of a head 26 which is fixed to the screw 22 and cooperates with a longitudinal guide or groove (not shown) of the casing so as to prevent rotation of the screw.
  • terms and expressions indicating positions and orientations such as “axial”, “longitudinal” and “transverse”, must be interpreted with reference to the axis of translation x of the screw 22.
  • the screw 22 is shown in an axially retracted and axially advanced position, respectively.
  • the axially retracted position corresponds to a tensioned condition of the Bowden cable and operation of the parking brake.
  • the axially advanced position corresponds instead to a released condition of the brake.
  • a helical spring 40 is arranged in a tubular cavity 32 of the casing, around the screw 22, between a shoulder 33 of the casing and an axially sliding sleeve 50.
  • the spring 40 is extended and urges the sleeve 50 into an axially retracted position against a stop 34 of the casing.
  • the transverse surface 25 of the screw is axially spaced from the sleeve 50.
  • Figure 4 shows an alternative embodiment of the invention, where the screw nut 21 is rotationally connected to a ring nut 121 also mating in a threaded manner with the screw 22.
  • a resilient thrust element 140 in the form of an annular disc, is arranged between the screw nut 21 and the ring nut 121.
  • the screw nut and the ring are connected by the gear wheel 12 of the reduction unit 11.
  • a limited capacity for relative axial sliding of the screw nut and the ring is envisaged so that, when there is no traction load acting on the screw during operation of the brake, the resilient disc 140 is able to expand elastically so as to move away axially the screw nut from the ring.
  • the resilient thrust exerted on the screw nut causes reversal of the flanks of the screws making mutual contact.
  • the force F 2 exerted by the resilient piece moves away from each other the flanks 21a, 22a which co-operate during operation of the brake, and at the same time moves towards each other the opposite flanks 21b, 22b. It will be noted that the reversal of the contact flanks does not occur for the ring nut 121.

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  • Engineering & Computer Science (AREA)
  • Transportation (AREA)
  • Mechanical Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Fluid Mechanics (AREA)
  • Transmission Devices (AREA)
  • Gripping Jigs, Holding Jigs, And Positioning Jigs (AREA)

Abstract

A friction screw mechanism (20) for a linear electromechanical actuator comprises a rotatable screw nut (21) and a translatable output screw (22). The screw and the screw nut have threads with respective first facing flanks (21a, 22a) which engage against each other when the mechanism is operated so as to impart to the screw (22) a translatory movement which produces a traction force (F1), and respective second facing flanks (21b, 22b) arranged on the other sides of the threads. The screw mechanism has, associated therewith, a resilient part (40, 140) able to exert on the screw or on the screw nut (21, 22) a thrust (F2) which moves axially the first flanks (21a) of the screw nut (21) away from the first flanks (22a) of the screw (22) and at the same time moves towards each other the second flanks (21b, 22b) of the threads. This relative movement of the screw and screw nut allows the lubricating grease situated at the interface between the threads to flow onto the first flanks, which work under load, and to areas where lubrication is of vital importance for the working life and efficiency of the mechanism.

Description

An improved friction screw mechanism
The present invention relates to an improved friction screw mechanism for a linear electromechanical actuator, in particular for tensioning and releasing a cable for operating a parking brake.
Braking devices associated with linear electromechanical actuators are known where the latter comprise an electric motor and a screw mechanism which converts the rotary movement imparted to the rotor of the electric motor into a linear movement of an output member which applies a braking force to the parking brakes of the vehicle by means of a Bowden cable.
With parking brake actuators, the friction screw mechanisms exert a very high traction force in only one of the two possible directions of displacement and therefore it is only one of the two flanks of the threads on the screw and on the screw nut which works with sliding friction under a high load.
It has been found that, with time, the grease designed to lubricate the threaded surfaces of the screw mechanism accumulates between the flanks which are not under load and is instead lacking or is absent on the flanks which are engaged under load. Inadequate lubrication of these surfaces reduces the working life of the actuator.
The object of the present invention is to provide an improved screw mechanism in order to overcome the abovementioned drawback.
This and other objects and advantages, which will be understood more clearly in the following, are achieved according to the invention by a screw mechanism having the features defined in the appended claims. In brief, the screw mechanism is associated with a spring member which, when the mechanism is in a condition where it does not produce a traction force for operating the parking brake, is able to impart an axial thrust to the screw or the screw nut such as to displace the screw and the screw nut relative to each other so as to move away from each other those facing flanks of the threads of these two members which co-operate with sliding friction when the mechanism produces the abovementioned traction force. This relative axial movement of the screw and screw nut allows the lubricating grease contained at the interface between the threads of the screw and the screw nut to flow along the flanks which work under load and where lubrication is of vital importance for the working life and efficiency of the mechanism.
The structural and functional features of two preferred, but non-limiting embodiments of a screw mechanism according to the invention will now be described; reference is made to the accompanying drawings, in which:
Figure 1 is a cross-sectional perspective view of an electromechanical actuator which includes a first embodiment of a screw mechanism according to the invention; Figure 2 is an enlarged axially sectioned view of a part of the screw mechanism according to Figure 1;
Figure 3 is a cross-sectional view of the part shown in Figure 2 in a different operating position;
Figure 4 is an axially sectioned view of a part of a second embodiment of a screw mechanism according to the invention.
With reference initially to Figure 1, designated overall at 10 is a linear electromechanical actuator intended to operate the parking brake (not shown) of a motor vehicle. The actuator 10 is associated with an electric motor (not shown) which, by means of a gear reduction unit 11 and an associated screw mechanism 20, imparts linear pulling and release movements to a Bowden cable (not shown) connected to the parking brake.
The screw mechanism 20 comprises a screw nut 21 rotatable about a longitudinal axis x and a threaded rod or screw 22 displaceable along the axis x. The screw nut 21 is rotatably supported inside a rigid casing 30 by a bearing 31 and receives the rotational movement from a gear 12 of the reduction unit 11. The screw mechanism converts the rotational movement of the screw nut 21 into a linear translation movement of the screw 22. The screw 22 has an output end 23, which can be connected to a Bowden cable (not shown), and an opposite end 24 which has a transverse surface 25. In the example shown, the transverse surface 25 forms part of a head 26 which is fixed to the screw 22 and cooperates with a longitudinal guide or groove (not shown) of the casing so as to prevent rotation of the screw. As used herein, terms and expressions indicating positions and orientations, such as "axial", "longitudinal" and "transverse", must be interpreted with reference to the axis of translation x of the screw 22.
hi Figures 2 and 3 the screw 22 is shown in an axially retracted and axially advanced position, respectively. The axially retracted position corresponds to a tensioned condition of the Bowden cable and operation of the parking brake. The axially advanced position corresponds instead to a released condition of the brake.
As schematically shown in Figure 2, when the brake is operated, the flanks 22a of the threads of the screw are engaged against the flanks 21a of the screw nut so as to impart to the screw an axial movement in the direction indicated by the arrow A. This is the "under load" working condition discussed in the introductory part of the description. In this condition, the lubricating grease accumulates in the gaps defined between the opposite flanks 21b, 22b of the threads. The arrow F1 indicates the axial load acting on the screw.
A helical spring 40 is arranged in a tubular cavity 32 of the casing, around the screw 22, between a shoulder 33 of the casing and an axially sliding sleeve 50. hi the operating condition shown in Figure 2, the spring 40 is extended and urges the sleeve 50 into an axially retracted position against a stop 34 of the casing. The transverse surface 25 of the screw is axially spaced from the sleeve 50. When the electric motor is activated so as to release the brake (Figure 3), the screw is moved forwards in the direction indicated by the arrow B. During this release operation, no significant load is applied to the screw. When the transverse surface 25 comes into contact with the sleeve 50, the spring 40 is compressed and the screw receives from the latter a force F2 which pushes the screw with respect to the screw nut in an axial direction such as to move away from each other the flanks 21a, 22a which co-operate during operation of the brake and, consequently, at the same time, move towards each other the other opposite flanks 21b, 22b. This relative axial movement of the screw and screw nut forces the lubricating grease to flow back into the gap between the facing flanks 21a, 22a which work under load. The lubrication of these threaded surfaces is thus ensured, as is also a longer working life of the entire screw mechanism and the actuator forming part thereof.
Figure 4 shows an alternative embodiment of the invention, where the screw nut 21 is rotationally connected to a ring nut 121 also mating in a threaded manner with the screw 22. A resilient thrust element 140, in the form of an annular disc, is arranged between the screw nut 21 and the ring nut 121. hi the example shown, the screw nut and the ring are connected by the gear wheel 12 of the reduction unit 11. A limited capacity for relative axial sliding of the screw nut and the ring is envisaged so that, when there is no traction load acting on the screw during operation of the brake, the resilient disc 140 is able to expand elastically so as to move away axially the screw nut from the ring. The resilient thrust exerted on the screw nut causes reversal of the flanks of the screws making mutual contact. As per the embodiment shown in Figures 1-3, in the variant according to Figure 4 also, the force F2 exerted by the resilient piece moves away from each other the flanks 21a, 22a which co-operate during operation of the brake, and at the same time moves towards each other the opposite flanks 21b, 22b. It will be noted that the reversal of the contact flanks does not occur for the ring nut 121. It will be understood, however, that, as a result of the movement of the screw, the re-lubrication of the flanks 21a, 22a also spreads onto the flanks 121a of the ring nut, and more generally, over all the threaded surfaces of the mechanism.

Claims

1. A friction screw mechanism (20) for a linear electromechanical actuator, in particular for tensioning and releasing a cable for operating a parking brake, wherein the mechanism comprises a first threaded member (21) which rotationally drivable about a longitudinal axis (x) and a second threaded output member (22) translatable along said axis as a result of rotation of the first threaded member (21), and wherein the two threaded members (21 , 22) have threads with respective first facing flanks (21a, 22a) which engage against each other when the mechanism is operated so as to impart to the threaded output member (22) a translation movement which produces a traction force (F1), and respective second facing flanks (21b, 22b) arranged on the other sides of the threads; characterized in that the screw mechanism has, associated therewith, a resilient means (40, 140) capable of exerting on one of the two threaded members (21, 22) a longitudinal resilient thrust (F2) such as to move axially the first flanks (21a) of the first threaded member (21) away from the first flanks (22a) of the second threaded member (22) and at the same time move towards each other the second flanks (21b, 22b) of the two threaded members (21, 22), so as to cause lubricating grease to flow from the second flanks (21b, 22b) onto the first flanks (21a, 22a) when the screw mechanism is not operated to produce said traction force (F1).
2. A screw mechanism according to Claim 1, characterized in that the first threaded member (21) is an axially stationary screw nut mounted rotatably inside a casing (30), the second threaded member (22) is an axially translatable screw and the resilient means (40) is capable of exerting the longitudinal resilient thrust (F2) on the screw (22).
3. A screw mechanism according to Claim 2, characterized in that the resilient means (40) is arranged between a fixed surface (33) of a casing (30) and a transverse surface (25) of the screw (22), and the screw is able to reach at least one axially advanced position in one direction (B) such as to cause the compression of the resilient means (40) and receive the longitudinal resilient thrust (F2).
4. A screw mechanism according to Claim 3, characterized in that the resilient means (40) is axially arranged in between the fixed surface (33) of the casing and a part (50) mounted axially slidably with respect to the casing (30) along the screw (22).
5. A screw mechanism according to Claim 3, characterized in that the resilient means is a helical spring seated in a tubular cavity (32) of the casing around the screw (22).
6. A screw mechanism according to Claim 1, characterized in that the first threaded member (21) is an axially stationary screw nut rotatably mounted inside a casing (30), the second threaded member (22) is an axially displaceable screw and the resilient means (140) is able to exert the longitudinal resilient thrust (F2) on the screw nut (21).
7. A screw mechanism according to Claim 6, characterized in that the resilient means (140) is arranged axially between the screw nut (21) and a ring nut (121) and mates in a threaded manner with the screw (22) and is rotationally connected to the screw nut (21) with a limited capacity for axial sliding with respect to the latter.
8. A screw mechanism according to Claim 7, characterized in that the resilient means (140) is an annular disc arranged axially between the screw nut (21) and a ring nut (121).
9. A screw mechanism according to Claim 7, characterized in that the screw nut (21) and the ring nut (121) are rotationally connected by a gear wheel (12) of a reduction unit
(11) associated with the screw mechanism.
PCT/IB2007/055284 2007-01-30 2007-12-21 An improved friction screw mechanism WO2008093178A2 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
ITTO20070064 ITTO20070064A1 (en) 2007-01-30 2007-01-30 SCREW MECHANISM WITH PERFECT FRICTION.
ITTO2007A000064 2007-01-30

Publications (2)

Publication Number Publication Date
WO2008093178A2 true WO2008093178A2 (en) 2008-08-07
WO2008093178A3 WO2008093178A3 (en) 2008-09-25

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/IB2007/055284 WO2008093178A2 (en) 2007-01-30 2007-12-21 An improved friction screw mechanism

Country Status (2)

Country Link
IT (1) ITTO20070064A1 (en)
WO (1) WO2008093178A2 (en)

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3303261A1 (en) * 1983-02-01 1984-08-02 Bengel, Brigitte, 7148 Remseck Device for transmitting longitudinal movements in a flexible or stiff guide sleeve
EP0236546A2 (en) * 1986-03-14 1987-09-16 FIAT AUTO S.p.A. A flexible transmission device, particularly for control devices

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3303261A1 (en) * 1983-02-01 1984-08-02 Bengel, Brigitte, 7148 Remseck Device for transmitting longitudinal movements in a flexible or stiff guide sleeve
EP0236546A2 (en) * 1986-03-14 1987-09-16 FIAT AUTO S.p.A. A flexible transmission device, particularly for control devices

Also Published As

Publication number Publication date
ITTO20070064A1 (en) 2008-07-31
WO2008093178A3 (en) 2008-09-25

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